设x=cos(2a),则:(1+x)=1+cos(2a)=2(cos(a))^2;so:(2(cos(a))^2)^3=5/2;(cos(a))^6=5/16;令cos(a)=b,即: b^6=5/16;然后用第莫弗定理求b,依次反推得x
