Find the sum of the first 56 terms of the AP
whose 4th term is 25 and 6 th term is 37.
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Answer:
Given→ a4 = 25
a6 = 37
.
a + 3d = 25 --------(1)
a + 5d = 37 --------(2)
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{ Using elimination method }
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a + 3d = 25
+a + 5d = 37
- - = -
___________
-2d = -12
d = 6
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now put d=6 in equation (1)
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a + 3(6) = 25
a + 18 = 25
a = 7
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sum of 56 terms =
s = n/2 [ 2a + (n-1) d]
s = 56/2 [ 2(7) + (56 - 1)7 ]
s = 28 [ 14 + 55(7) ]
s = 28 [ 14 + 385]
s = 28 [399]
s = 11,172
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