MathNotes · User Manual

1Product overview

A pen, a page, an answer.

MathNotes brings "calculation" back to "writing" itself: you no longer switch back and forth between a notes app and a calculator, copying and pasting. In one note, words and expressions live together, with results inline and live, recalculating as variables change. It is for:

Grocery lists, splitting bills, household budgeting;
Students doing homework, organizing formulas, converting units;
Engineers / freelancers jotting drafts and estimating on the spot;
Anyone who wants to "write and calculate at the same time."

Key features

⚡

= at line end

Add = at the end of an expression and the orange result appears instantly, clear even next to long text.

🔗

Linked variables

Define a variable once; change it in one place and every expression referencing it recalculates.

📐

Engineering math

Degree-mode trig, scientific functions, and full SI unit conversion built in.

🧮

Built-in calculator

A standard four-operation calculator with smart percent, sign toggle, and repeat =.

🌗

Theme & font

Light / Dark / System; note body font 14–30, applied live.

🔒

Zero-permission privacy

No network, no account, no ads — your data stays on your device.

2Install & first launch

Search "MathNotes" or "数学笔记" in app stores.

Install MathNotes, package com.zgt.mathnotes, by JUJUMI TECH.
On first launch, 10 pre-filled "life + engineering" example notes are injected (fixed ids seed-01 … seed-10), covering household budget, dinner split, trip sharing, motor reduction, resistor divider, and more. Tweak the numbers to try, or long-press to delete. Example notes are multilingual and follow your system language on first launch (Simplified Chinese, English, 日本語, 한국어, Français, Deutsch, Español, العربية), falling back to English for unsupported languages.
Tap + at the bottom right to create a new note and start writing.
To learn everything: open Settings → About → How to use notes and try as you read.

💡 Existing notes from users who upgraded (had an older version) are not overwritten; and if you delete all notes, the examples will not reappear — the first-launch flag, once written, is permanent.

3The interface

The app has four main screens.

① Notes list (home)

Apple Notes style: white rounded shadow cards, grouped automatically by date (Today / Yesterday / Older). Long-press for multi-select, search from the top. Top-right opens Settings and the Calculator.

② Note editor

Two areas: a title area on top (large bold), and a body area below. Expressions in the body ending with = show an inline orange result; the top bar has a font-size (A) button to adjust the body font, plus a Save button. Tapping Save persists immediately; when you go back, if there are unsaved changes, a dialog asks "Save & exit / Discard changes / Cancel".

③ Calculator

iOS style: round buttons, orange operators, light-gray background, with a press scale animation and haptic feedback. A History button sits in the top bar. Only standard four operations (+ - × ÷ % ± =); no scientific mode.

④ Settings

Includes: Appearance (theme switch), Data & Backup (import / export), and About (version, privacy statement, developer contact, how to use notes).

4Inline calculation

This is the app's core capability.

On any line, type an expression and end it with =; the answer appears immediately:

rent + coffee × 30 =

gives the orange result 3635.0. Supported:

Arithmetic + - * / (multiply can be ×, divide can be ÷);
Parentheses ( );
Percent, e.g. 20 + 20 × 15% = → 23;
Negatives, decimals, and powers ^, e.g. 2^10 = → 1024.

✍️ Smooother writing: spaces are auto-added around = as you type — x= becomes x = for tidier layout; a space is also added between a trailing number and a system unit (e.g. 100km → 100 km). Non-system units (e.g. xyz) are treated as plain text and never misread as a unit.

Results are shown highlighted in orange, with the number in orange and the unit in gray for clearer hierarchy. Change a number earlier and the result on that line updates.

5Variables

Turn the whole note into a living document that calculates itself.

Define a variable

On its own line, assign with name = value:

coffee = 4.5
distance = 100 km
price = 25 yuan

Any text after the value on the right of = (units, notes, etc.) is treated as a label and ignored — no more errors. If the trailing token is a single system unit (e.g. km, yuan), it is remembered as that variable's unit and carried downstream: after distance = 100 km, distance × 2 = shows 200 km directly.
Grayed units: in results the number is orange and the unit is gray; system units you type at line end are auto-grayed too.

Use variables

coffee × 5 =
rent + coffee × 30 =

Linked recalculation

Change coffee = 4.5 to coffee = 6, and the results of both lines above update automatically.

⚠️ Naming tip: the system already recognizes v (volt), n (newton), a (ampere) as units. To avoid misreads, examples and engineering use Chinese variable names (e.g. "电压"/voltage, "电机转速"/motor speed, "路程"/distance). Also, a calculation line should not start with a Chinese label (e.g. 输出转速电机转速÷减速比 = would have its variable name eaten by the leading-strip rule and error out) — start directly with a variable / number / operator.

Composite units derived automatically

When an expression mixes units with × / ÷, the result unit is assembled by dimension — speed shows km/h, rotation shows rpm or m/min; you can also write a composite unit directly (e.g. m/s) and it is recognized and kept. No manual assembly needed.

6Units & conversion

Connect the source unit and target unit with → (or ->); the result carries the target unit directly.

100 km to mi =        62.14 mi
25 °C to °F =         77 °F
5 kw to w =           5000 W
2 a to ma =           2000 mA
1 mol → mmol =       1000 mmol
300 m/min → m/s =    5 m/s

Supports common units for length, area, volume, mass, temperature, speed, and time, plus a full set of SI units:

Base quantities

meter(m), kilogram(kg), second(s), ampere(a, milli ma/micro µa/kilo ka), kelvin(k), mole(mol, milli mmol/kilo kmol), candela(cd).

Derived quantities

hertz(hz/kHz/MHz/GHz), newton(n/kN/mN), watt(w/kW/mW/GW), coulomb(c/mC/µC/kC), volt(v/mV/kV/µV), farad(f/mF/µF/nF/pF), ohm(Ω/ω/ohm, kilo kΩ/milli mΩ), weber(wb), tesla(mt), henry(mh), becquerel(bq), gray(gy), sievert(sv), electron-volt(ev/keV/MeV/GeV), and more.

Temperature °C / °F / K uses affine conversion (via kelvin); other dimensions use linear "factor → base unit → divide back".
µ (micro) and u are synonyms for easy input; units are case-insensitive (e.g. KW and kw are equivalent).

⚠️ Conversion syntax: the left side must be a number literal (e.g. 300 m/min → m/s); you cannot use a variable name (e.g. conveyor distance → m/s will not convert). To convert a variable, write a new line with a number literal.

7Math engine

Beyond arithmetic, there is engineering-grade math.

7.1 Trigonometry (degree mode)

Use degrees directly — no need to convert to radians first. The degree suffix ° treats the value as degrees in trig:

sin(30°) =         0.5
sin(deg(30)) =     0.5
cos(60°) =         0.5

⚠️ Common mistake: without °, the value is in radians, so sin(30) is not 0.5; to get 0.5 write sin(30°) or sin(deg(30)).

7.2 Scientific functions

Supported: sqrt (square root), sin cos tan asin acos atan, sinh cosh tanh, exp, ln (natural log), log (common log), log2, abs, sign, round floor ceil, cbrt (cube root), hypot, mod (modulo), deg/rad (degree/radian conversion), and more.

sqrt(16) =         4
log(1000) =        3
ln(2.71828) =      1
hypot(3,4) =       5

7.3 Very large / very small numbers

Results that are too large or too small switch to scientific notation automatically — no unreadable long strings of digits.

8Built-in standard calculator

When you don't want to open a note, use the built-in standard four-operation calculator.

Percent: 100 + 5% = → 105;
Sign toggle: switch between positive and negative with one tap (±);
Smart repeat:
- Press = repeatedly to keep adding: 5 + 3 = = = → 8, 11, 14;
- Press = right after an operator to repeat the last operand: 5 + = → 10.
Live preview: the current result shows in light gray above the expression as you type; ⌫ deletes one digit at a time;
After = the result shows right after the equals sign and the preview disappears; typing a new digit starts a fresh calculation.

📌 The calculator is standard four operations: use scientific functions, trig, and unit conversion in inline notes, not on the calculator page.

9Theme & display

🌗 Theme

In Settings → Theme, switch between Light / Dark / System. In dark mode results highlight in orange, easy on the eyes at night.

🔠 Note font size

Tap the font-size (A) icon in the editor top bar; drag the slider or step with A-/A+ (range 14–30), with live preview and auto-save; the title scales 1.4× in sync.

10Data & backup

Your data belongs only to you.

Local storage

Notes persist as a local JSON file (notes.json), written atomically (temp file then rename) to avoid data loss if a write is interrupted. No network / cloud dependency at all.

Import / Export (SAF)

Path: Settings → Data & Backup → Export notes / Import notes, using the Android system file picker (SAF) — no cloud involved.

Export: writes all notes to a JSON file you choose (UTF-8, formatted for readability).
Import: reads from a JSON file you choose and merges by id — overwrites existing, adds new; it never deletes local notes that are absent from the file.

System backup & device switch

Notes are auto cloud-backed-up / restored to a new device via Android system backup (depending on your system account settings).

Trash (anti-misdelete)

A confirmation appears before deleting (both list multi-select delete and in-editor delete). Deleted notes go to the Trash (the trash icon in the notes list top bar, next to Settings), where you can restore or delete forever, or empty the trash with one tap. Permanent deletion is unrecoverable; trash data is also local only.

📌 In the current version the calculator history is in-memory and not kept after restart (persistence arrives in a later version). For important data, rely on system backup or periodically keep a JSON via "Export notes".

11Settings reference

The Settings page has four sections.

① Appearance

Theme dropdown: Light / Dark / System, applied immediately.

② Data & Backup

"Export notes" and "Import notes" — tapping either triggers the system file picker (see section 10).

③ About

Shows the app name, version (1.4.14 · build 27), privacy statement, and developer info:

Developer: JUJUMI TECH;
Email kefu@jujumi.com.cn (tap to copy to clipboard with a hint);
Website (tap to open the browser, or copy the URL if that fails).

④ How to use notes

A greatly upgraded in-app guide, grouped as "Basic calculation / Variables & units / Unit conversion / Scientific functions & angles / Typing tips", covering line-end = instant calc, cross-line variables, Chinese units (米/kg/秒/°C/m²/亩…), conversion (to), composite-unit derivation, ASCII x as multiply, root restoring dimension, and more — try as you read, the fastest way to get started.

12Example notes (10)

Injected on first launch to demo core features; use them as templates by changing the numbers.

Life scenarios (5)

Monthly household budget

Sum rent/utilities/food/transport/fun, then compute monthly savings. Demonstrates multi-line accumulation and subtraction.

Dinner split (AA)

Dishes + drinks subtotal, (subtotal)×0.1 service fee, then ÷6 per person. Demonstrates parentheses and percent.

Trip cost sharing

Flights 2×1280, hotel 4×360, tickets 3×120, summed then ÷4. Demonstrates chained multiply and splitting.

Mortgage payment ratio

monthly payment ÷ monthly income × 100 gives the percentage of income.

Savings goal

Gap between goal and saved, then ÷ per month to get months remaining. Demonstrates goal breakdown.

Engineering scenarios (5)

Motor reduction ratio

motor speed = 1500 rpm, motor speed ÷ reduction ratio = → 300 rpm. Demonstrates variables + rpm unit.

Conveyor speed conversion

distance ÷ time = gives 300 m/min, then 300 m/min → m/s = → 5 m/s. Demonstrates unit conversion.

Resistor divider

voltage = 12 V, voltage×5÷(10+5) = → 4 V. Demonstrates a divider circuit and the V unit.

Average driving speed

distance = 120 km, distance ÷ time = → 80 km/h. Demonstrates the speed unit.

Ramp length

vertical height = 3 m, vertical height ÷ sin(30°) = → 6 m. Demonstrates degree-mode trig.

13Tips & tricks

Define once, link everywhere: define often-changing values (exchange rate, unit price, hourly rate) as variables and reference them throughout; tune by editing one line.
Budget template: use variables for a "monthly budget" — define rent, food, transport, then rent+food+transport = for the total.
Dinner AA: (hotpot+milk tea+drinks)÷4 = splits evenly.
Conversion cheat-sheet: before a trip, write 100 km to mi, 25°C to °F in a note for reference.
Engineering estimates: sqrt, hypot, log2 inline — no need to switch to a scientific calculator.
Dark mode for the eyes: at night, enable dark theme; the orange results stay clear and gentle.
Font by scenario: on the subway, bump the font to 26–30 so it's readable one-handed.
Examples as templates: the 10 examples can be reused by changing numbers — no need to start from scratch.

14FAQ

What can it calculate?

Arithmetic, parentheses, percent, variables, plus degree-mode trig, scientific functions, and unit conversion. Very large/small results use scientific notation automatically.

How is it different from a notes app + calculator?

Ordinary tools keep notes and calculator separate, so you switch and copy-paste. MathNotes keeps math and text in one document, with inline results that recalculate with variables.

Are my notes safe?

The app has zero permissions, is fully offline, and has no account — notes stay only on your device, never via any server or third-party SDK.

Which platforms are supported?

Currently Android; search "MathNotes" or "数学笔记". Package com.zgt.mathnotes.

Can I change the font size?

Yes. Note body font 14–30, applied instantly and auto-saved; title scales 1.4×.

How do I convert units?

Connect with to (or →), e.g. 100 km to mi, 25 °C to °F. The left side must be a number literal.

Is calculator history saved?

Not in the current version; persistence is planned for a later release.

Can I delete the example notes? Will they come back?

Yes, long-press to delete; even if you then clear all notes, the examples will not be re-injected (the first-launch flag is already written).

15Troubleshooting

Symptom	Possible cause	Fix
No result at line-end `=`	The line is not a valid expression / contains full-width punctuation	Check parentheses and that operators are half-width; remove extra characters and retry
Result shows `Error`	Divide by zero / function domain error (e.g. `sqrt(-1)`, `log(0)`)	Fix the expression; `log` must be >0, `asin` input must be ∈[-1,1]
Variable not working	Variable name has spaces/Chinese, or wasn't defined as `name=value`; or the calc line starts with a Chinese label	Write the variable line as `coffee=4.5` and reference with the same name; start calc lines with a variable / number
Unit conversion does nothing	Not written as `from→to`, or unit unsupported	Use `→`; full SI units are built in; temperature uses `°C/°F/K`; left side must be a number
Error after writing a unit on assignment	Old versions treated the unit as an expression and errored	Fixed: `name=value unit` treats the unit as a label to ignore (e.g. `distance=100 km`)
`sin` result is wrong	Degrees mistaken for radians	Add `°` or wrap with `deg()`: `sin(30°)`
Changed a variable but result didn't update	Recalc not triggered / IME suggestion	Edit that line a bit, or switch notes and come back
Font size adjustment not saved	Didn't use the top-bar A popup	Use the editor top-bar "font size" icon; it auto-saves on release
Calculator history gone	History is in-memory in the current version	Expected; not kept after restart, persistence planned later
Can't find the app	On third-party channels / restricted region	Android only

16Privacy & security

MathNotes is designed on the principle that "you are the customer, not the product":

🔌 Zero permissions

Requests no camera, contacts, storage, network, or any other permission.

📴 Offline

Fully usable offline; no server is collecting your notes.

🚫 No account

Open and use; no registration or login.

💎 No ads

Open and use; no ads.

Data stays on your device only: notes are saved only on your device, never via any server or third-party SDK.

17Contact & feedback

Developer: JUJUMI TECH

Website: jujumi.com.cn

Email: kefu@jujumi.com.cn

For feature suggestions or bug reports, include your version number and a screenshot and send to the email above.

This manual matches the app's actual behavior; if the interface differs slightly from the description, the in-app "Settings → About → How to use notes" prevails. Current version: v1.4.14 (versionCode 27).

18Industry calculation examples

Every example below can be run for real inside a MathNotes note; each group defines variables first, then writes calculation lines whose results recalculate live as variables change.

💡 For unit conversion use to (or →) and the left side must be a number literal; a composite unit (e.g. m³/s) may show only the number without the unit — that is expected behavior.

① Daily life & personal finance

rent = 3500
coffee = 4.5
rent + coffee × 30 =          3635
orig = 200
discount = orig × 0.8 =        160
principal = 10000
rate = 0.05
years = 3
amount = principal × (1 + rate)^years =   11576.25
height = 1.75 m
weight = 70 kg
BMI = weight ÷ height^2 =       22.8571429

② Construction & renovation

len = 5 m
wid = 4 m
area = len × wid =            20 m²
h = 3 m
volume = len × wid × h =        60 m³
radius = 3 m
circle area = π × radius^2 =     28.2743339 m²
top = 4 m
bottom = 6 m
trap h = 5 m
trapezoid = (top + bottom) × trap h ÷ 2 =   25 m²
bricks = 20 m² ÷ (0.6 m × 0.6 m) =     55.5555556

③ Mechanical

mass = 10 kg
acc = 9.8 m/s²
force = mass × acc =           98 N
vert = 3 m
ramp = vert ÷ sin(30°) =         6 m
speed = 100 km/h
time = 2 h
distance = speed × time =        200 km

④ Electrical & electronics

volt = 12 V
amp = 2 A
res = volt ÷ amp =            6 Ω
power = volt × amp =           24 W
series = 100 Ω + 220 Ω =        320 Ω
parallel = 1 ÷ (1/100 Ω + 1/220 Ω) =  68.75 Ω
divider = 12 V × 100 Ω ÷ (100 Ω + 200 Ω) =  4 V
res2 = 1000 Ω
cap = 1 µF
tau = res2 × cap =            0.001 s
fc = 1 ÷ (2 × π × res2 × cap) =    159.154943 Hz

⑤ Hydraulics & pneumatics

10 MPa to bar =              100 bar
force = 1000 N
area = 0.01 m²
pressure = force ÷ area =         100000 Pa
flow v = 5 m/s
section = 0.02 m²
flow = flow v × section =         0.1

⑥ Machining & manufacturing

stroke = 0.05 m
feed = 0.0002 m/s
mach time = stroke ÷ feed =       250 s

⚠️ Some units (e.g. mm, cm³, mL) have known limitations in arithmetic dimensional conversion; prefer base units (m, kg, s, A, V, Ω, W, Hz, N, Pa, J) first, then use to to convert to the unit you want — results are most stable this way.

A notebook that does math——end a line with =, get the result

1Product overview

Key features

= at line end

Linked variables

Engineering math

Built-in calculator

Theme & font

Zero-permission privacy

2Install & first launch

3The interface

① Notes list (home)

② Note editor

③ Calculator

④ Settings

4Inline calculation

5Variables

Define a variable

Use variables

Linked recalculation

Composite units derived automatically

6Units & conversion

Base quantities

Derived quantities

7Math engine

7.1 Trigonometry (degree mode)

7.2 Scientific functions

7.3 Very large / very small numbers

8Built-in standard calculator

9Theme & display

🌗 Theme

🔠 Note font size

10Data & backup

Local storage

Import / Export (SAF)

System backup & device switch

Trash (anti-misdelete)

11Settings reference

① Appearance

② Data & Backup

③ About

④ How to use notes

12Example notes (10)

Life scenarios (5)

Monthly household budget

Dinner split (AA)

Trip cost sharing

Mortgage payment ratio

Savings goal

Engineering scenarios (5)

Motor reduction ratio

Conveyor speed conversion

Resistor divider

Average driving speed

Ramp length

13Tips & tricks

14FAQ

15Troubleshooting

16Privacy & security

🔌 Zero permissions

📴 Offline

🚫 No account

💎 No ads

17Contact & feedback

18Industry calculation examples

① Daily life & personal finance

② Construction & renovation

③ Mechanical

④ Electrical & electronics

⑤ Hydraulics & pneumatics

⑥ Machining & manufacturing

A notebook that does math
——end a line with =, get the result