f(x)=cosx(1-cos^2x)=cosx(sin^2x)
f^2(x)=2cos^2x(sin^2x)(sin^2x)/2<=(2cosx^2x+2sin^2x)^3/54=4/27
所以f(x)<=2√3/9
a^2(3+b^2)=2a^2(b^2+3)/2<=(2a^2+b^2+3)^2/8=2