y=tanx-tanx^3/(1+2tanx^2+tanx^4)的最大值与最小值的积是

y = (tanx - tan³x)/(1 + 2tan²x + tan^4x)= (tanx - tan³x)/(1 + tan²x)²= (tanx - tan³x)/(sec²x)²= (tanx - tan³x) * cos^4x= sinxcos³x - sin³xcosx= (...sec²x是什么东西？