- #1
pc2-brazil
- 205
- 3
Homework Statement
Suppose a and b are real numbers with 0 < b < a. Show that, if n is a positive integer, then
[tex]a^n - b^n \leq na^{n-1}(a-b)[/tex]
Homework Equations
The Attempt at a Solution
I'm trying to show this by induction.
Let P(n) be the proposition that [itex]a^n - b^n \leq na^{n-1}(a-b)[/itex]
I've already verified that P(1) is true, which completes the basis step.
Inductive step:
I must show that, if P(k), then P(k+1).
So, I first assume that this is true for an arbitrary k: [itex]a^k - b^k \leq ka^{k-1}(a-b)[/itex]
Then, I must show that, if P(k) is true, it follows that [itex]a^{k+1} - b^{k+1} \leq (k+1)a^k(a-b)[/itex].
This is where I'm having trouble.
I'm trying to find that [itex]a^{k+1} - b^{k+1}[/itex] is less than or equal to an expression involving [itex]a^k - b^k[/itex], so that I can use the expression for P(k) to derive an inequality for [itex]a^{k+1} - b^{k+1}[/itex].
I've tried several ways, like trying to rewrite [itex]a^{k+1} - b^{k+1}[/itex] as [itex]aa^k - bb^{k}[/itex] and then writing that [itex]aa^k - bb^k \geq aa^k - ab^k = a(a^k - b^k)[/itex] (since a > b), but this doesn't help, because I'm looking for something that [itex]a^{k+1} - b^{k+1}[/itex] is less than or equal to, not greater than or equal to.
Thank you in advance.