Posted by **CMM** on Sunday, October 24, 2010 at 11:17pm.

If f(x)= sec x, find f"(Pi/4)

I am not sure how to take the 2nd derivative?

f'(x)= sec x tan x

f"(x)=???

Is it f"(x)= (sec x tan x)(sec^2x)???

Please Help!

- Calc -
**Reiny**, Sunday, October 24, 2010 at 11:24pm
f'(x)= sec x tan x is correct

now use the product rule

f''(x) = secx(sec^2x) + tanx(secx)(tanx)

= sec^3 x + secx(tan^2x)

I will leave the subbing to you

- Calc -
**Chelsea**, Monday, October 25, 2010 at 1:55am
What do you mean subbing? I thought that was the final answer? In the book it leaves f"(x)= - cos x for f(x)= cos x

- Calc -
**Chelsea**, Monday, October 25, 2010 at 2:08am
Oops, sorry about that. Do you mean this:

sec^3(Pi/4) + sec(Pi/4)(tan^2(Pi/4)) =

3*sqrt(2)

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