由x^2 + y^2 = 9 ,我们可假设x = 3cosa ,y = 3sina ,a 属于（0,2π）,于是有a = 3cosa + 3根号3 sina = 3( cosa + 根号3 sina) = 6(cos60°cosa + sin60°sina) = 6cos(60°-a)于是,我们可明显地知道a的最大值与最小...