# Help with algebra for a proof: manipulating an expression

#### restin84

I have a proof that I have to do.

for all positive natural numbers n and for any integers a and b such that a != b a^n - b^n is divisible by a-b

the induction hypothesis is to assume the a^k - b^k is divisible by a-b

I need to show that a^(k+1) - b^(k+1) is divisible by (a-b). I have seen the solution floating around on the internet. The expression can be manipulated into the form
a(a^k - b^k) + b^k(a -b) which is divisible by (a-b) both directly in the second term and by the induction hypothesis in the first. I don't necessarily want to know how to get to this result. But how do I THINK about how to get to the result?

#### MarkFL

Re: Help with algebra for a proof: manipulating an expressio

Obviously, the base case $$\displaystyle P_1$$ is true, so the induction hypothesis $$\displaystyle P_n$$ is:

$$\displaystyle a^n-b^n=k(a-b)$$

Now, my inclination is to multiply both sides by $$\displaystyle (a+b)$$:

$$\displaystyle \(a^n-b^n$$(a+b)=k(a-b)(a+b)\)

Now arrange as follows:

$$\displaystyle a^{n+1}-b^{n+1}=k(a-b)(a+b)-ab\(a^{n-1}-b^{n-1}$$\)

And the rest follows from applying the induction hypothesis to the second term on the right.

#### restin84

Re: Help with algebra for a proof: manipulating an expressio

MarkFL said:
Obviously, the base case $$\displaystyle P_1$$ is true, so the induction hypothesis $$\displaystyle P_n$$ is:

$$\displaystyle a^n-b^n=k(a-b)$$

Now, my inclination is to multiply both sides by $$\displaystyle (a+b)$$:

$$\displaystyle \(a^n-b^n$$(a+b)=k(a-b)(a+b)\)

Now arrange as follows:

$$\displaystyle a^{n+1}-b^{n+1}=k(a-b)(a+b)-ab\(a^{n-1}-b^{n-1}$$\)

And the rest follows from applying the induction hypothesis to the second term on the right.
So does this mean I should be using strong induction then? since $$\displaystyle a^{n-1} - b^{n-1}$$ is less than $$\displaystyle a^{n} - b^{n}$$?