# Finding the slope of the tangent line using the f(a+h)-f(a)/h form?

1. Jan 26, 2014

### JessicaJ283782

Find the slope of the tangent line to the curve f(x)=x-x^3 at the point (1,0)

So, I went through and plugged (a+h) in everywhere I saw "x"

(a+h)-(a+h)^3

Factoring:

(a+h-a^3-3a^2h-3ah^2-h^3)

I'm really stuck on what to do next...I don't see anything you can cancel/pull out? I know you need to cancel the h in the denominator, but I can't pull an h out because of the "a"s...

Thank you!

2. Jan 26, 2014

### Dick

That's just the f(a+h) part. Subtract the f(a) part before you start trying to factor.

3. Jan 26, 2014

### JessicaJ283782

But isn't the f(a) part 0?

4. Jan 26, 2014

### Ray Vickson

Yes, in this case. But you have not used that information yet in your calculation of f(a+h).

Last edited: Jan 26, 2014
5. Jan 26, 2014

### JessicaJ283782

So can I plug in 0 for a? and then factor out h, and then plug in 0 for h?

6. Jan 26, 2014

### Ray Vickson

Don't ask; just decide for yourself.

7. Jan 26, 2014

### JessicaJ283782

But you can't because then you're going to get 0/0, right?

8. Jan 26, 2014

### CAF123

What do a and h represent in your equation?

9. Jan 26, 2014

### JessicaJ283782

a would represent the "lim as x->a" so it would be the "lim as x->1"...h is what you are trying to factor out.

10. Jan 26, 2014

### Dick

If you are saying a=1 in this problem, that would be right.

11. Jan 26, 2014

### JessicaJ283782

But I calculated the slope another way, and the answer is -2. When I do it this way, you'll get 0/0 though if you plug in 1 for a because you would then plug in 0 for the h's and have no number left?

12. Jan 26, 2014

### Dick

Show your work. You should be able to cancel an h and then let h->0 and get -2 this way too.

13. Jan 26, 2014

### JessicaJ283782

I put the first steps on how I factored it out above

Then,

a+h-a^3-3a^2h-3ah^2-h^3

Plug in 1:
h-3h-3h^2-h^3

pull out an h, I get h(-3-3h-h^2)

Then you'll get 0/0?

14. Jan 26, 2014

### Dick

i) your expression will have an h in the denominator as well. Cancel it with the one in the numerator before taking h->0. ii) Your algebra isn't correct h-3h-3h^2-h^3 isn't equal to h(-3-3h-h^2). Fix it.

15. Jan 26, 2014

### Ray Vickson

There is something fundamental that you are not "getting". Here is how it should go.

(1) First you calculate [f(a+h)-f(a)]/h, where h is NOT zero. Simplify it as much as you can.
(2) Then (and only then), let h → 0 to see what is the limit.

You never actually set h = 0. Taking h → 0 is not the same as setting h = 0.