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.