It is proved that if the intersection of straight lines passing through point e and AD is f,

△ADE folds along point e, and point a coincides with point d.

. . △AFE?△DFE

Ae = ed: af = FD ∴ f is the midpoint of AD, that is, EF is the midline of △AEF.

∵ EF⊥AD ∴ △AEF is an isosceles right triangle.

< < a is 45.

Ps: It seems certain ~ ~ ~ Then ps: This sogou really has everything (O. O) ||||||| Always feels that he is hurting his friends.