In an A.P the 5th term is 30 and 12th term is 65 .what is the sum of first 20
terms.
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a + 4d = 30
a + 11d = 65
Solve it by elimination
4d - 11d = -35
-7d = -35
d = 5
a + 4(5) =30
a = 30 - 20
a = 10
S20 = 20/2{2(10) + 19(5)}
= 10{20 + 95 }
= 10(115)
= 1150
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