a^n-b^n=(a-b)(a^(n- 1)+a^(n-2)b+a^(n-3)b^2+a^(n-4)b^3+......+a^(n-(n-2))b^(n-3)+a^(n-(n- 1))b^(n-2)+a^(n-n)b^(n- 1))

Where a n represents the n power of a, and so on.

So Bn-1= (b-1) (b (n-1)+b (n-2)+b (n-3)+...+b 2+b1+.

Then dividing by b- 1 equals (b (n-1)+b (n-2)+b (n-3)+...+b 2+b1).

Multiplied by b (n+ 1)+ 1 on the left is equal to b2n+b (2n-1)+b (2n-2)+...+b (n+1)+b (n-).