Need help finding derivative

1. Oct 1, 2012

physics=world

1. f(x) = 3cosx - ((sinx)/(4))

2. Relevant equations
use the rules of differentiation

3. i tried adding them together which i got

((12cosx - sinx) / (4))

then, i used quotient rule

((4)(-12sinx - cosx) - (12cosx - sinx)(0))/ (4^2)

which i got this for an answer

(-12sinx - cosx) /( 4)

but the answer is
(-3sinx - cosx) /( 4)

Last edited by a moderator: Oct 1, 2012
2. Oct 1, 2012

Staff: Mentor

Neither of these is correct, assuming that the problem is as you posted.

If you are learning the quotient rule, you should also have learned some simpler rules, such as the sum rule and the constant multiple rule.

As a matter of course, you should never use the quotient rule if the denominator is a constant. It's not wrong to do so, but since the quotient rule is a bit on the complicated side, there is a greater chance of getting the derivative wrong due to an error of some sort.

3. Oct 1, 2012

physics=world

ohh. thanks i see what i did wrong.