已知π/4已知π/4
已知π/4设t=tan^2x-1,则t>0,所以y=tan2xtan^3x=tan^3x(2tanx)/(1-tan^2x)=2tan^4x/(1-tan^2x)=2(t+1)^2/(-t)=-2(t+1/t+2)≤-8..