# Weird Derivative problem

1. Oct 29, 2008

### asdfsystema

1. The problem statement, all variables and given/known data

2. Relevant equations
Trig identities or straightforward derivative?

3. The attempt at a solution

1. C
2. B
3. A
4. D

I tried taking the derivative of these, but a friend of mine here told me it deals with trig identities and isn't a derivative equation at all. Is he wrong ?

I am pretty sure these are the solutions but i want to double check ...

2. Oct 29, 2008

### Staff: Mentor

Your friend is wrong. You hae to take the derivatives of the four given functions, and then match the derivatives you got with the answers shown by using trig identities. For 1, C is correct. For the rest, use trig identities to simplify the answers and compare your results with the derivatives you got for the given functions.

3. Oct 29, 2008

### asdfsystema

2. y= tan(x)
y'= sec^2(x)
trig identity used: 1+tan^2(x)= sec^2(x)

3. y=cos(tan(x))
y'= -sec^2(x)*sin(tan(x))

4.y=sin(x)tan(x)
y'=cos(x)*tan(x)+sec^2(x)*sin(x)

Is this correct? Thank you for the quick response by the way.

4. Oct 29, 2008

### Staff: Mentor

Yep, yep, and yep.

5. Oct 29, 2008

### asdfsystema

Thank you ! problem solved