Fill in any two of the three sides (a, b, c) of a right triangle to solve the third — a² + b² = c².
Fill in exactly 2 of the 3 fields — the 3rd is solved for.
The Pythagorean theorem relates all three sides with one equation, so knowing any two sides lets the calculator solve for the third. Filling in all three (or only one) leaves either no unknown to solve for, or too many unknowns for one equation.
In a right triangle, the hypotenuse (the side opposite the right angle) is always the longest side. If the value you entered for c isn't greater than the leg you provided, the three lengths can't form a valid right triangle, so the calculator flags the error instead of returning an imaginary result.
Angle A is found from the inverse sine of (leg a / hypotenuse), since sin A = opposite/hypotenuse in a right triangle. Angle B is simply 90° − Angle A, because the two non-right angles in a right triangle always add up to 90°.