# Homework Help: Finding the second derivative

1. Feb 12, 2010

### illegalvirus

1. The problem statement, all variables and given/known data

Find the exact value of f''(2) if f(x)=$$\sqrt{3x-4}$$

2. Relevant equations

See above

3. The attempt at a solution

I've tried to use the product rule to differentiate.

f= x(3x -4)$$^{\frac{1}{2}}$$

f'= (3x -4)$$^{\frac{1}{2}}$$ + $$\frac{3}{2}$$$$^{\frac{-1}{2}}$$

f''= $$\frac{3}{2}$$x(3x -4)$$^{\frac{-1}{2}}$$ . $$\frac{3}{2}$$x(3x -4)$$^{\frac{-1}{2}}$$ + (3x -4)$$^{\frac{1}{2}}$$ . $$\frac{-9}{4}$$x(3x-4)$$^{\frac{-3}{2}}$$

= $$\sqrt{3x -4}$$ . $$\frac{-9x}{4\sqrt{(3x-4)^{3}}}$$

Now I am kind of stuck here. I've tried it a few times and I have no idea what I am doing wrong, some advice please?

2. Feb 12, 2010

### Anti-Meson

3. Feb 12, 2010

### illegalvirus

Somehow I got the same answer

f'(x) = $$\frac{3}{2}$$x(3x -4)$$^{\frac{-1}{2}}$$
f''(x) = $$\frac{-9}{4}$$(3x -4)$$^{\frac{-3}{2}}$$

4. Feb 12, 2010

### Anti-Meson

Your first derivative is incorrect. However, you have somehow ended up with the correct second derivative :S.

f''(x) = $$\frac{-9}{4}$$(3x -4)$$^{\frac{-3}{2}}$$

[BTW: f'(x) = $$\frac{3}{2}$$(3x -4)$$^{\frac{-1}{2}}$$ ]

Last edited: Feb 12, 2010
5. Feb 12, 2010

### Staff: Mentor

Why are you writing your function like this? There is no factor of x outside the radical, so the product rule is not called for here. This is your function.
$$f(x)=\sqrt{3x-4}$$

6. Feb 12, 2010

### illegalvirus

Sorry, I think the first derivative is this,

f'(x) = $$\frac{1}{2}$$x(3x -4)$$^{\frac{-1}{2}}$$

7. Feb 12, 2010

### Anti-Meson

Please read your text book on chain rule. It is wrong to put an x in the front or a half for that matter.

8. Feb 12, 2010

### illegalvirus

Ahh, whoops. The original equation to derive is actually $$f(x)=x\sqrt{3x-4}$$!

9. Feb 12, 2010

### Anti-Meson

Ok, in that case you were right to use the product rule.

f'(x) = $$\frac{3x}{2}(3x-4)^{-0.5}+(3x-4)^{0.5}$$

I leave the rest to you.

Last edited: Feb 12, 2010
10. Feb 12, 2010

### illegalvirus

Wait, how did you get that?

11. Feb 12, 2010

### Anti-Meson

Product rule.
$$\frac{d[u(x)v(x)]}{dx} = u\frac{dv}{dx} + v\frac{du}{dx}$$

12. Feb 12, 2010

### illegalvirus

Okay thank you, I've just realized that I've forgotten to multiply by the derivative of the brackets.

13. Feb 12, 2010

### Anti-Meson

Just in case you get stuck the answer is:

$$3(3x-4)^{-0.5}-\frac{9}{4}x(3x-4)^{-1.5}$$

Though you should work this out by yourself. To verify.

14. Feb 12, 2010

### illegalvirus

I got this, $$\frac{3}{2}(3x-4)^{-0.5}-\frac{9}{4}x(3x-4)^{-1.5}$$

But when finding the exact value of $$f''(2)$$ I had a bit of a problem as well.

So far I have, $$f''(2)= \frac{-9}{4\sqrt{2}} + \frac{3}{2\sqrt{2}}$$.

$$=\frac{-6\sqrt{2}}{4\sqrt{2}}$$

But the answer is $$\frac{3\sqrt{2}}{8}$$

15. Feb 13, 2010