This is from HSC 2004, q7 Show that for all positiver integers n, x[ (1+x)^n-1 + (1+x)^n-2 +...+ (1+x)^2 + (1+x) + 1 ] = (1+x)^n-1 ii) Hence show that for 1< and= k < and= n, (n-1) + (n-2) + ... + (k-1) = (n) (k-1) (k-1) (k-1) (k) ( ) ( ) is nCr (the combinations thing) I can't do the second part. Help pls EDIT: ok, doesn't format properly. So yea, if someone has ext 1 2004 q7b, can someone explain part 2??