# Math Help - Derivative HELP!!

1. ## Derivative HELP!!

show that the curve y=6x3+5x-3 has no tangent line with slope 4.

2. ## Re: Derivative HELP!!

Take the derivative, equate to 4, and what do you find?

3. ## Re: Derivative HELP!!

if you do y'(4) it equals 293 so is that the proof?

4. ## Re: Derivative HELP!!

No, you aren't being asked to evaluate $y'(4)$, you are being asked to show that there is no real $x$ such that:

$y'(x)=4$.

5. ## Re: Derivative HELP!!

so how do you do that? whats the difference?

6. ## Re: Derivative HELP!!

First, take the derivative of y with respect to x, then set that equal to 4 and solve for x. What do you find?

edit: I realize this is the same thing I said in my first post, but to say any more would be to essentailly give away the problem, and this would do you little good.

7. ## Re: Derivative HELP!!

well if you take the derivative and set it equal to 4 and solve you get plus or minus sqrt(-1/18) which is a nonreal answer so is that why there is no tangent line with the slope 4?
Originally Posted by MarkFL2
First, take the derivative of y with respect to x, then set that equal to 4 and solve for x. What do you find?

edit: I realize this is the same thing I said in my first post, but to say any more would be to essentailly give away the problem, and this would do you little good.

8. ## Re: Derivative HELP!!

Yes, you've hit the nail on the head!