+75 Let r be a real number such that |r| < 1. Express
\sum_{n = 0}^{\infty} n*r^n*(n + 1)*(n + 2)
in terms of r.
+111 The sum is
6r(1−r)4the fraction with numerator 6 r and denominator open paren 1 minus r close paren to the fourth power end-fraction
6𝑟(1−𝑟)4
