Question

Find the sum of series 6^3+7^3+………………..…..+20^3.

(a) 43875

(b) 83775

(c) 43775

(d) 43975

This question was addressed to me in an online quiz.

The question is from Sum to n Terms of Special Series in division Sequences and Series of Mathematics – Class 11

Answer 1

Correct choice is (a) 43875

To elaborate: 6^3+7^3+………………..…..+20^3

= (1^3+2^3+3^3+……..+20^3) – (1^3+2^3+3^3+4^3+5^3)

= (20*21/2)^2 – (5*6/2)^2

= (210)^2 – (15)^2

= 225*195

= 43875.