FIND X2+6+18+.............+x=531440
a=2; R=3sum to nth term = a(R^n-1)/(R-1)so531440 = 2*(3^n-1)/(3-1)= 3^n - 13^n = 531441n = 12x = 2x3^(12-1)=2x3^11= 354294
