Analysis: Connect EB, construct a right triangle, let AE be X, then DE=BE=4-x, and get a linear equation about X by pythagorean theorem.

6.20 1 1 Lishui

7.20 1 1 Yueyang

Analysis:

(1) Whether it is correct can be judged directly according to Pythagorean theorem;

② Proof of congruence condition △ ABF △ EDF can be obtained by folding;

③ Using the nature of congruent triangles can solve the problem;

④ In ④Rt△ABD, the definition of trigonometric function can be used to judge whether it is correct or not.

④ In Rt△ABD, ∠ ADB ≠ 45, ∴AD≠BD? Cos45, so the statement is wrong.

So the correct one is ② ③.

So choose B.

8.20 1 1 Jinan

Analysis: Let the three sides of a right triangle be A, B and C, respectively, to show the area comparison of the triangle.

So choose a.