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