Let r be the positive real solution to x^3 + 2/5 x - 1 = 0. Find the exact numerical value of r^2 + 2r^5 + 3r^8 + 4r^11 + ....
r = ~0.86756
∑[ n*0.86756^(3n -1), n, 1, ∞] = ~6.25
