Sum of first 20 tems of an AP is 1/3 of the next 20 terms. If first term is 1. Find the sum of first 30 terms.
using sum(n) = (n/2)(2a + (n-1)d) and a = 1
sum(20) = 10(2 + 19d) = 20 + 190d
sum(40) = 20(2 + 39d) = 40 + 780d
sum of next 20 terms = sum(40) - sum(20)
= 40+780d - 20 - 190d = 20 + 590d
20+190d = (1/3)(20+590d)
60 + 570d = 20+590d
40 = 20d
d= 2
sum(30) = 15(2 + 29d) = 30 + 870 = 900