Finding c
Rolle's theorem and the Mean Value Theorem say that f′(x) takes
on a certain value at some point c between a and b. But how
do you find such a c? Is there only one?
Example 1: Suppose that f(x)=x3−6x2+5x. Plugging
in, we see that f(0)=0 and f(1)=0. (The third root is x=5,
by the way). By Rolle's theorem, there has to be a c between 0
and 1 with f′(c)=0. What is it?
Solution: To find c, we first compute f′(x)=3x2−12x+5. So 0=f′(c)=3c2−12c+5. We then solve this with
the quadratic formula: c=12±√(12)2−4(3)(5)6=12±√846=6±√213 Since we are looking for something between 0 and
1, we want c=(6−√21)/3≈0.4725 instead of
(6+√21)/3≈3.5275. (If we had used Rolle's theorem
with a=1 and b=5, we would have wanted the larger value of
c.)
This answer isn't a particularly nice number. But nothing guarantees
that real-world problems will have simple answers!
Example 2: Consider the function f(x)=ex between x=1
and x=3. Find a point where the line tangent to the curve y=f(x)
is parallel to the line between (1,e) and (3,e3). The
x-value of this point is the c in the MVT.
DO: Try to find the value
of c before reading further.
Solution: This is the Mean Value Theorem with f(x)=ex,
a=1 and b=3. We solve it in steps:
- We compute f(b)−f(a)b−a=e3−e2.
- We look for c. Since f′(x)=ex, f′(c)=ec=e3−e2, so c=ln(e3−e2)≈2.1614.
- Once we have c, we have our point (c,ec)=(ln(e3−e2),e3−e2)≈(2.1614,8.684).
|