3sinα-2cosα=0 所以 cosα=3sinα / 2(5sinα^3+cosα)/(2cosα^3+sinα^2cosα) = (5sinα^3+3sinα / 2)/[2(3sinα / 2)^3+sinα^2(3sinα / 2)]= (5sinα^2+3/2)/[27(sinα)^2 / 4 + 3sinα^2 / 2]= 20 / 33 + ...