# Find the slope of the tangent line

1. Dec 13, 2009

### Centurion1

1. The problem statement, all variables and given/known data
Find the slope of the tangent line using a specific formula

g(x)=3t-t2

at (0,0)

2. Relevant equations

Im told to use this equation by the book
f(c+deltax) - f(c)
Deltax

3. The attempt at a solution
Everytime i plug it in by way of the books style i get 2c. and then you are supposed to plug in the x value which gives me 0. But the right answer is 3

2. Dec 13, 2009

### rochfor1

Perhaps if you show some of your algebra...

3. Dec 13, 2009

### Centurion1

sorry.

yeah im not so sure its an aritmetic mistake but a misunderatanding of how to do things. but following what the book said to do here goes,

f(c + deltax) - f(c)
delta x

((c + deltax)2 - 3t) - (c2 -3t) foil it out
delta x

2c(deltax) + (deltax)2 simplify
Delta x

and then my book gets rid of both of the delta x's
i assume by simplyfying and i can only assume the deltax2 by plugging in zero because the lim approaches 0

and i get the wrong answer because i end up with 2c (which the book says to do) and then plug in a point which is 0 and its supposed to end up being 3?

4. Dec 13, 2009

### rochfor1

You're getting the wrong answer because $$f(c + \Delta x) \neq ( c + \Delta x )^2 - 3t$$. You should think about why.

5. Dec 13, 2009

### Centurion1

thats the problem the book gives an example for linear problems, eg y= x and parabolas, y=x^2

im not sure what to do with this?

6. Dec 13, 2009

### rochfor1

This isn't anything that would be an example in a calculus book, because it's function evaluation. (Although it may be in one of the "introductory" sections.)

If you have a function f(t) what does $$f(c + \Delta x)$$ mean? It means everywhere in the definition of f that you see a t, you should put a $$c + \Delta x$$.

7. Dec 13, 2009

### Centurion1

really? its in chapter two of my calculus book section 2.1 finding the slope for a tangent line.

so your saying it should look like

3(c + Delta x) - (c + Delta x)2

so

3c + 3Delta x - c2 + 2c(deltax) + (deltax)2

?

8. Dec 13, 2009

### rochfor1

I know this problem is a calculus problem, I'm saying the issue you're having isn't a calculus issue, so it might not be addressed in the examples.

Yes. You need to put $$c + \Delta x$$ everywhere you see t. Now do some algebra to simplify...

9. Dec 13, 2009

### Centurion1

oh okay i understand what your saying about what im doing wrong. i realize that f(x) is meant to plugged in whenever you see x. but the formulas was throwing me off.

but is this over delta x like the equation?

10. Dec 13, 2009

### rochfor1

Yes, the definition of derivative stays the same.

11. Dec 13, 2009

### Centurion1

so you can cancel a single delta x right? what happens with the other delta x's i assume the best one to cancel is the 2c delta x

sorry i know im making this harder than it should be.....