# Find c in (a, b) so tangent passes through 0

• April 8th 2010, 06:38 PM
Raied
Find c in (a, b) so tangent passes through 0
Let a; b be real numbers such that 0 < a < b. Let f be a function continuous on [a, b]
and dif erentiable on (a, b). Assume that f satis es the property
f(a) = f(b) = 0 and f'(a) = 0:
Show that there exist c in (a; b) such that the tangent to f at c passes through 0.

Any help? I am totaly lost.
• April 8th 2010, 06:39 PM
dwsmith
Sounds like the Mean Value Theorem
• April 9th 2010, 05:19 AM
Raied
hmm or could it be Rolles Theorem...
Anyon else?
• April 9th 2010, 05:48 AM
dwsmith
When you wrote $f'(a)=0$, was that supposed to be $f'(c)=0$

If you meant $f'(c)=0$, then it is Mean Value.

It is just different to see $f'(a)=0$ since that doesn't come up in either theorem.
• April 9th 2010, 06:18 AM
Raied
No it is actually f'(a)...
That is why I am getting confused. I have never seen that before
• April 9th 2010, 06:38 AM
Plato
The tangent line is $y=f'(c)x-cf'(c)+f(c)$.
To pass through (0,0) we must have $f(c)-cf'(c)=0$.
Consider the function $\frac{f(x)}{x}$ apply Rolle's Theorem.
• April 9th 2010, 07:13 AM
Raied
is x = a?
• April 9th 2010, 07:52 AM
Plato
Suppose that $g(x)=\frac{f(x)}{x}$ then $g(a)=g(b)=0$.
Moreover, on $(a,b)$ $g'(x) = \frac{{xf'(x) - f(x)}}{{x^2 }}$ so if $g'(c)=0$ then $cf'(c) - f(c)=0$.