How do you find the length of one side if you have the others on a triangle?

If the triangle is a right triangle, where one angle is equal to 90o, Then you can use the Pythagorean Theorem. To use this, label each side of the right triangle a, b, and c, in which c is the hypotenuse/longest side/side opposite of the 90o angle; a2+b2=c2 . Just input the values into this equation and you are able to get one side as long as you know the other two.

If the triangle is not a right triangle, then you would need to know an angle of one of the sides already known in order to use the law of cosines.