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How many terms of A.P 2, 7, 12, 17, addupto 990? |
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Answer» Let n terms of AP add upto 990 Sum of n terms of APSn = n/2(2a + (n - 1)d) Given a = 2, d = 7-2 = 5 Then,990 = n/2(2*2 + (n - 1)5)990 = n/2(4 + 5n - 5)990*2 = n(5n - 1)5n^2 - n - 1980 = 05n^2 - 100n + 99n - 1980 = 05n(n - 20) + 99(n - 20) = 0(5n +99)(n - 20) = 0n = - 99/5, 20 Therefore sum of 20 terms of given AP is 990 |
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