In an ap, the sum of the first n terms is 5n^2/2+3n/2. Find its 20th terms
Anjali , 8 Years ago
Grade 10
10 grade maths> In an ap, the sum of the first n terms is...
1 Answers
Arun
Sn = (5n2 + 3n) / 2
Therefore, S20 = [5 (20)2 + 3 (20) ] / 2
Thus S20 = 2060 / 2 = 1030
S19 = [5 (19)2 + 3 (19)] / 2 = 1862 / 2
Thus, S19 = 931
Tn = Sn - Sn-1
Therefore, T20 = S20 - S19
T20 = 1030 - 931 = 99
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