# Thread: which rule to use for this equation?

1. ## which rule to use for this equation?

y = x^4 - 4x^3 - 2 cos (x) + tan (x)

question asks to differentiate the following function

the answer i believe is 4x^3 - 16x^2 + 2 sin (x) + tan (x)

do i have the correct answer?

2. Originally Posted by JontyP
y = x^4 - 4x^3 - 2 cos (x) + tan (x)

question asks to differentiate the following function

the answer i believe is 4x^3 - 16x^2 + 2 sin (x) + tan (x)

do i have the correct answer?
no, the derivative of tan(x) is not tan(x)

why do you feel tan(x) should stay the same?

3. oops i missed out the last bit of the answer!

it should look like this
4x^3 - 12x^2 + 2 sin (x) + (1/cos^2(x))

i've changed the -16x^2 to a -12 too

4. Originally Posted by JontyP
oops i missed out the last bit of the answer!

it should look like this
4x^3 - 12x^2 + 2 sin (x) + (1/cos2(x))

i've changed the -16x^2 to a -12 too
that's correct now. by the way, we can write $\displaystyle \frac 1{\cos^2 x}$ as $\displaystyle \sec^2 x$