Y=tan2X*tan³X
＝2tan^4 x/(1-tan^2 x)
令t＝tan^2 x>1,
y=2t^4/(1-t^2)
=[2(t^2+1)(t^2-1)+2]/(1-t^2)
=-2(t^2+1)+2/(1-t^2)
=-2[(t^2-1)+1/(t^2-1)+2]
≤-2[2+2]
＝－8
当(t^2-1)＝1/(t^2-1),t＝√2时,y max＝－8