ABCD is a rectangle with AC =10 cm and BC=8cm. Diagonals AC and BD intersect at E and D2=20°. Calculate the sizes of all interior angles and AD,AE and AB? So can I say D1+D2=90° and what's the reason?

Yes, you can say that D1 + D2 = 90°, and the reason is that the diagonals of a rectangle bisect each other and form right angles. Therefore, D1 and D2 are complementary angles, and their sum is equal to 90°.