Question

Let S_n denote the sum of the first n terms of an A.P. If S_2n

= 3S_n, then S_3n: S_n is equal to-

• A. 4
• B. 6
• C. 8
• D. 10

Answer 1

Answer: (B)

6

Answer 2

S_2n = 3S_n

[2a + (2n –1) d] = $\frac { 3 n } { 2 }$ [2a + (n –1) d]

2a = (n +1) d

$\frac { S _ { 3 n } } { S _ { n } }$ = $\frac { \frac { 3 n } { 2 } [ 2 a + ( 3 n - 1 ) d ] } { \frac { n } { 2 } [ 2 a + ( n - 1 ) d ] }$= = 6