令y=sinα²cosα两边平方：y^2=(sinα)^4*(cosα)^2=[(1-(cosα)^2]^2*(cosα)^2=(1/2)[1-(cosα)^2][1-(cosα)^2][2(cosα)^2]（再利用平均值不等式：a,b,c∈R+ abc≤[(a+b+c)/3]^3 ）(1/2)[1-(cosα)^2][1-(c...