The question is this: because in trapezoidal ABCD, AD‖BC and diagonal AC⊥BD, if AD = BC = 7, what is the sum of trapezoidal ABCD areas?

The extension line from DE//AC to BC is in BD of E, DE = AC, DE⊥, BE = 10, s- trapezoid ABCD = S-RT triangle DBE.

That is, the triangle with the largest DBE area of Rt triangle, such as waist RT, has a waist length of 10 * 2/2 = 5√2.

Is it? The maximum area of trapezoid ABCD =1/2 * (√ 2) 2 = 25.