1. Because it is folded, DE=BE. Let AE=x, then BE=DE=(a-x). In Rt△ADE, AE is obtained by pythagorean theorem. +AD？ =DE？ What about x? +b？ =(a-x)？ , expand, a? -B? =2ax。

In a regular pentagon, ∠ c 'da = 108, ∠ ade = 108-90 = 18, while in Rt△ADE, AE = ad tan ∠ ade,

That is, x = b tan 18, then a? -B? =2a b tan 18