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The nth term of an ap is given-4n+15 . Find the sum of its 20 term |
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Answer» Thanks to all of you On putting n = 1, 2,3 ,…. in eq (1),For n = 1(an) = - 4n + 151 = - 4(1) + 15 a1 = 11\xa0For n = 2a2 = - 4(2) + 15 a2 = - 8 + 15 a2 = 7\xa0Common Difference , d = a2 - a1 d = 7-11 d = - 4\xa0By using the formula ,Sum of nth terms , Sn = n/2 [2a + (n – 1) d]S20 = (20/2)[2 × 11 + (20 -1)(-4)]S20 = 10 [22 + 19 × - 4[S20 = 10 [22 - 76]S20 = 10 × - 54S20 =\xa0- 540 Hence, the sum of first 20 terms of AP is - 540. nth term of an A.P,\xa0(an) = - 4n + 15……..(1)On putting n = 1, 2,3 ,…. in eq (1),For n = 1(an) = - 4n + 151 = - 4(1) + 15 a1 = 11For n = 2a2 = - 4(2) + 15 a2 = - 8 + 15 a2 = 7Common Difference , d = a2 - a1 d = 7-11 d = - 4By using the formula ,Sum of nth terms , Sn = n/2 [2a + (n – 1) d]S20 = (20/2)[2 × 11 + (20 -1)(-4)]S20 = 10 [22 + 19 × - 4[S20 = 10 [22 - 76]S20 = 10 × - 54S20 =\xa0- 540 Hence, the sum of first 20 terms of AP is - 540.\xa0 Kis book ka h Given : nth term of an A.P,\xa0(an) = - 4n + 15……..(1)\xa0On putting n = 1, 2,3 ,…. in eq (1),For n = 1(an) = - 4n + 151 = - 4(1) + 15 a1 = 11\xa0For n = 2a2 = - 4(2) + 15 a2 = - 8 + 15 a2 = 7\xa0Common Difference , d = a2 - a1 d = 7-11 d = - 4\xa0By using the formula ,Sum of nth terms , Sn = n/2 [2a + (n – 1) d]S20 = (20/2)[2 × 11 + (20 -1)(-4)]S20 = 10 [22 + 19 × - 4[S20 = 10 [22 - 76]S20 = 10 × - 54S20 =\xa0- 540 Hence, the sum of first 20 terms of AP is - 540. |
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