What is i^71 + i^65????

-i + i
0

i^1 = i
i^2 = -1
i^3 = -i
i^4 = 1
x^[a+b] = x^a * x^b
So...
i^71 = 1^[68 + 3] = i^68 * i^3 = 1 * -i = -i
i^65 = i^[64 + 1] = i^64 * i^1 = 1 * i = i
i^71 + i^65 = 0

Should be 6 x 10^-13 or 0.0000000000006