How do I do this problem?

Use Rolle's theorem to prove that, regardless of the value b, there is at most one point in the interval -1 <= x <= 1 for which x^3 - 3x + b = 0

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- Jun 7th 2007, 05:25 PMBrianWRolle's Theorem
How do I do this problem?

Use Rolle's theorem to prove that, regardless of the value b, there is at most one point in the interval -1 <= x <= 1 for which x^3 - 3x + b = 0 - Jun 7th 2007, 06:49 PMThePerfectHacker
We need to understand this problem carefully, because when I was first reading it, it seemed as if it said "there is at least one point which is a zero".

We want to show that there is**at most**one point. We will argue by contradiction. Say there are more than two points on . Pick any two distinct ones with . Define the function . Note that . Consider this function on the closed interval . We notice that , the function is continous on , and differenciable on . By Rolle's theorem we can find so that . Meaning . But that is impossible because since . Hence a contradiction. That means there can be at most one zero.

Q.E.D.