a=sin^2(36°)
b=sin^2(72°)=cos^2(36°)
∴a+b=1
1/a+1/b=(a+b)/(ab)=1/(ab)
(ab)^(1/2)=sin(36°)cos(36°)
=(1/2)(sin(36°)cos(36°)+cos(36°)sin(36°))
=(1/2)sin(72°)
∴ab=(1/4)sin(72°)^2

n=1のとき
(a^(-1)+b^(-1))(a+b)
=((a+b)/(ab))(a+b)
=(a+b)^2/(ab)=1/(ab)=4/sin(72°)^2＞1