Question

Find the sum of 20 terms of the series 3 + 6 + 10 + 15 + …..

Answer 1

3 + 6 + 10 + 15 + …

(1 + 2) + (1 + 2 + 3)+ (1 + 2 + 3 + 4)+ ….. = 1 +

(1 + 2) + (1 + 2 + 3)+ (1 + 2 + 3 + 4)+ ….. −1

Required sum = Sum of 21 terms of series 1 + (1 + 2) + (1 + 2 + 3)+.....-1