# Very simple derivative

1. Nov 6, 2008

### yoleven

1. The problem statement, all variables and given/known data
3/x^2

when I take the quotient rule ,
I get:
(0*x^2-3*2x)/(x^2)^2
isn't that -6x/x^4 or -6/x^3
My calculator says -6 and so it is, but why and what am I missing?

2. Nov 6, 2008

### kmikias

ok From what i see you are right.the answer is -6x^-3.

but at what point are you evalusting.did they give you a point or did you just type on your calculator.you have to have value of x.to type on your calculator

3. Nov 6, 2008

### yoleven

I originally had to differentiate f(t)=cos^-1(3/t^2)
my answer was f'(t)=-1/sqrt(1-(3/t^2)^2)*-6t
f'(t)=6t/sqrt(1-9/t^4)
I got the "-6t" part wrong, it was supposed to be just -6.

when I differentiate just 3/x^2 into my calculator and it comes out -6
on my ti89 calc. I go to math, calculus, differentiate and type in:
d(3/x^2,x) and get -6

4. Nov 6, 2008

### jimvoit

Instead of using the quotient rule, you might find it easier to evaluate
3 x^-2

5. Nov 6, 2008

### yoleven

Oky, thanks. I see my problem.
I should have looked into the mirror sooner, the problem was there all along!