Maths
Three geometry solvers you type onto the drawing itself, and a circle that fills in from whichever of its four boxes you use. Where your numbers determine nothing, the solvers leave the boxes blank rather than guess.
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- Circle Calculator Enter radius, diameter, circumference or area — get the other three.
- Triangle Calculator Type into the drawing — angles and sides fill themselves in.
- Quadrilateral Calculator Two triangles on a shared edge — a quadrilateral, or a triangle cut by a line.
- Regular Polygon Calculator Pick a polygon, type one measurement, and read the rest off the drawing.
Arithmetic you can check
Every answer arrives with its working: the formula, your numbers substituted, and a result to six significant figures — eight in the circle. The formula shown is the textbook one; the arithmetic is sometimes rearranged. A third side from two sides and the angle between them is evaluated as hypot(b − c, 2√(bc)·sin(A÷2)): on a sliver with a hundredth-of-a-degree apex, the law of cosines cancels two near-equal numbers into noise.
Angles are in degrees, and the exact cases come back exact. In double precision the sine of 180 degrees is 1.2246e-16 rather than nought, and the cosine of 90 degrees is 6.1e-17. Unguarded, the first divides into a circumradius of 1e16, and the second sends the SSA branch the wrong way on a right angle.
Figures you type into
In the triangle, the quadrilateral and the polygon, the boxes are ordinary HTML fields placed as a percentage over a vector drawing rather than inside it. SVG's foreignObject would be the obvious way, and is used nowhere here: it scales its contents with the viewBox, so every box's text would swell and shrink as the figure refitted.
The fit runs once per change, never on its own result: labels are a constant size in screen pixels, so rescaling moves them and alters the box being fitted, and a fit that re-enters itself makes a drawing breathe under a motionless cursor. The polygon is fitted to the circle through its corners, so changing the side count slides them round a circle that stands still.
The limits, stated
Three of the four take no unit at all. A shape keeps its angles whether its sides are millimetres or miles, so the answers hold as long as you stay consistent; a unit menu would only invite converting your figures by accident. Only the circle carries units, and its area is written in that unit squared.
The precision you type is read as a claim. An interior angle of 128.57° is accepted as a heptagon, whose angle is 128.5714…°; 128.5° is refused, because to one decimal place that is a different assertion. The narrowing stops at one part in a thousand million: seventeen decimals would assert 5e-18, finer than a double. Nothing here rearranges algebra or plots a function.