Explanation and example

First check that three positive lengths satisfy the triangle inequality. Add the sides to find the perimeter. Heron’s formula gives area from the semiperimeter, and the law of cosines gives the angles in degrees. All sides must use the same length unit.

Example: sides 3, 4 and 5 form a right triangle. Its perimeter is 12, its area is 6 square units, and the angle opposite side 5 is 90°. The results can be checked with the Pythagorean relation.

Limits

Sides 1, 2 and 3 form a degenerate figure and are rejected. The area unit is the square of your chosen length unit. The diagram explains the labels and should not be used to measure a real object.