-2+4-8+16-32 Those are the powers of 2 with alternating signs, So they are the powers of (-2). So, like your example: 1,2,3,4,5 is i -2+4-8+16-32 (-2)1 = -2 (-2)2 = 4 (-2)3 = -8 (-2)4 = 16 (-2)5 = -32 Rule for the nth term is nth term = (-2)n Edwin