Question

If the sum of 1st 7terms of an AP is 49 and that of 17 terms is 289.Find sum of 12 terms
​

Answer 1

Answer:

s7 = 49

7/2 [2a+(7-1)d) = 49

2a + 6d = 49/7 ×2

2a + 6d = 14—1

s17 = 289

17/2 [2a+(17-1)d] = 289

2a + 16d = 289/17 ×2

2a +16d = 17×2

2a+16d = 34—2

using elimination method in 1,2

2a + 6d = 14

2a+16d = 34

- - -

—–—–————

-10d = -20

d = 2

put value d in 1

2a + 6d = 14

2a + 6×2 = 14

2a + 12 = 14

2a = 14-12

2a= 2

a = 1

s12 = 12/2 [2×1 + (12-1)2]

= 6[2 + 11×2]

= 6(2+22)

= 6×24

= 144

follow me on brainly