4-3+(9/4)-(27/16)+(81/64)-

i think i need to figure out the sigma notation first, then off to solve the sum with using the calculator

but the problem is that i can't even find the pattern to write the notation

the rule An = A1 + (n-1)(d) doesn't work in this problem b/c the number is NOT constantly increasing

anyone could give a hand to help me out here?