Thought I was finally getting the hang of these... What am I doing wrong? (trig integral)

Question

hartnn · Answer

question?

anonymous · Answer

yeah... I'll upload an image :p

hartnn · Answer

everything seems to be correct!
just add +c in the end

hartnn · Answer

why do u feel u are wrong?

hartnn · Answer

oh, wait sec^2 x = 1+ tan^2 x

anonymous · Answer

ohhhhhhh.... I think that's my problem.... I messed up my trig identities. What is the correct identity?

hartnn · Answer

so there will be + sign in between
1+tan^2 x = sec^2 x

anonymous · Answer

ah ok :). Thank you. I need to commit these to memory... I'm not too good with trig identities... Also, there's one integral that I haven't tried yet that is  int_sin(3x)cos(5x)... Is there some kind of identity to solve that integral?

hartnn · Answer

sin A cos B = ?

hartnn · Answer

sin A cos B = 0.5*(sin(A+B)+sin (A-B))
now can u solve ?

anonymous · Answer

Will that work even if the two quantities inside the trig functions are different?

hartnn · Answer

A and b are different...

anonymous · Answer

ok thanks :). I'm about to do that problem, so I will see how it goes :P. Thank you for the help too. I've lost count of how many times you've helped me haha.

hartnn · Answer

welcome :)
no need to keep count.