In Arithmatic series, the 7th and the 9th numbers are 55 and 71 respectively. Then what is
the sum of first 12 numbers ?
(1) 512 (2) 521 (3) 612 (4) 721
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Step-by-step explanation:
a + 6d = 55
a + 8d = 71
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2d = 16
d = 8
a = 55 - 48 = 7
S(12) = 12/2 {2(7) + (12-1)8}
= 6 { 14 + 88}
= 6 × 102 = 612
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