in an A.P if the sum of its 4th and 10th terms is 40, and the sum of its 8th and 16th terms is 70, find the sum of its first 20 terms
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a4 + a10=40
a + 3d + a + 9d = 40
2a + 12d = 40 - --------(1)
a8 + a16 = 70
a + 7d + a + 15d = 70
2a + 22d = 70 --------(2)
(2) - (1)
2a + 22d = 70
2a + 12d = 40
___________
0a + 10d = 30
d= 30/10 = 3
sub d = 3 in (1)
2a + 12 x 3 = 40
2a + 36 = 40, 2a = 4
a = 2
S20 = 20/2 [ 2a + (20-1) d ]
=10 [ 4 + 19 x 3 ]
=610.
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275.
This is the required answer to this question.
This is the required answer to this question.
leosinghrajput:
sry
2650
610
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