Use the Pythagorean theorem for a right triangle: a² + b² = c²
Rearrange the formula to solve for the missing side
If the hypotenuse is missing: c = √(a² + b²)
If a leg is missing: a = √(c² – b²) or b = √(c² – a²)
Use the Law of Sines for non-right triangles when you know an angle and its opposite side
Use the Law of Cosines when you know two sides and the included angle, or all three sides except one
Use the triangle angle sum: angles add to 180°
Use similar triangles if the triangles are proportional
Use perimeter if the missing side can be found from the total perimeter
Check that the side lengths satisfy the triangle inequality
Verify the units and calculate carefully