計唔到既二階導數問題.

y= tan^2 (x)
dy/dx = 2tanx sec^2 (x)
d^2y/dx^2 = 2sec^2 (x) (sec^2 (x)) + 2tanx [2sec(x) * sec(x) tan(x)]
= 2 sec^4 (x) + 4 sec^2 (x) tan^2(x)
= 2sec^2 (x) [ sec^2(x) + 2 tan^2 (x)]
= 2(1+y)(1+y+2y) [Note that sec^2 (x) = 1 + tan^2 (x) = 1+y]
= 2(1+y)(1+3y)