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Which term of A.P. 121, 117, 113, ... is its first negative term? If it is the nth term, find Sn. |
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Answer» .P. 121, 117, 113.......a=121 d= 117-121= (-4)let the nth term of the given AP is 1ST negative term then, N<0a+(n-1)d < 0=>121+(n-1)(-4) < 0=>121-4n+4 < 0=>125-4n < 0=>n > -125/-4= 31.25since the 1st negative term is greater than 31.25it is 32ND termSn= n/2( 2a+(n-1)d = 32/2 (2×121+(32-1)(-4) =16(242+31×-4) =16× 242-124 = 16×118 = 1888 |
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