Question

If the sum of first 6 terms is 12 and sum of first 10 terms is 60, then

find the sum of its n terms.

Answer 1

The sum of n terms = A + (n-1)d = = 2n - 5

Step-by-step explanation:

Given data:

S 6 = 12

6/2(A + 6th term) = 12

3(A + A +(n-1)d) = 12

3(2A + (n-1)d) =12

2A+(n-1)d = 4

2A + 5d = 4 --------- (1)

S 10 = 60

10/2(2A+9d) = 60

2A + 9d = 12 ---------- (2)

• By elimination method:

2A+5d = 4

2A+9d = 12

__________

- 4d = -8

d = 2.................. (3)

• Substituting equation (3) in equation (1)

2A +5d = 4

2A + 5(2)=4

2A = 4-10

A = -3

• Sum of n terms = A + (n-1)d

= -3 + (n-1)(2)

= -3 + 2n -2

= 2n - 5

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Answer 2

Answer:

Step-by-step explanation: