The Diagonal of a Rectangle

A diagonal cuts a rectangle into two right triangles. The diagonal is the hypotenuse of each, so the Pythagorean theorem gives its length from two neighbouring sides.

Here triangle ABC has a right angle at B and legs 8 and 6: ${d}^{2} = {8}^{2}+{6}^{2} = 100$, so the diagonal is 10. If the two sides you know are not in the same triangle, use opposite sides first: in triangle ACD the legs are CD and DA, and CD = AB.