Hi there, i am having problems evaluating

$\displaystyle

F(x)=tan (x^2)$

which is differentiated to...

$\displaystyle

2x sec^2 (x^2)

$

when

$\displaystyle

x=\pi /4, for f'

$

when i put the numbers in i get the wrong answer, can someone explain what i am doing wrong, also im not sure what to do with the sec^2, how do you work this out on a calculator (for future reference) ???

Many thanks