About letx

letx ("let x = …") is a formula reference where every formula is something you can use, not just read.

Most formula sheets give you the formula and leave the rest to you. Here, each of the 100 formulas has a calculator that solves for whichever letter you are missing and shows the working line by line, a worked example, the mistakes people commonly make, and, where a picture helps, a graph or shape you can change with sliders.

It covers what a student meets from about age 14 to the first year of university: algebra, geometry, coordinate geometry, trigonometry, sequences and series, probability and statistics, finance maths, the basics of calculus, and the equations of motion.

How it is made

letx is built and kept up to date with automated tooling and rebuilt every week. Every build re-runs the full test suite and recomputes every worked example; see how the maths is checked. The site is static, fast and works offline once visited. It sets no cookies and has no trackers or advertising.

It is not affiliated with any exam board, school or publisher. The finance formulas are maths, not financial advice: real loans and savings products add fees and their own rate conventions.