
Combinatorial proof help
I'm lost. Could someone explain to me how I would approach these problems?
1a) Prove this equation combinatorially
$\displaystyle 2 \times 3^0 + 2 \times 3^1 + 2 \times 3^2 + ... + 2 * 3^{n1} = 3^n  1$
Next, solve this equation using ordinary base10 numbers:
$\displaystyle 2 \times 10^0 + 2 \times 10^1 + 2 \times 10^2 + ... + 2 * 10^{n1} = 10^n  1$
1b) Let a and b be positive integers with $\displaystyle a > b$. Give a combinatorial proof of the identity $\displaystyle (a + b)(a  b) = a^2  b^2$.