Question

in an ap the sum of the first n terms is 5n^2/2 + 3n/2. find its 20th term

Answer 1

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Sum of two terms = 5(4)/2 + 3(2)/2 = 10+3 = 13 . Second term = 13 - 4 = 9 . Common difference = 9 - 4 = 5 . 20 th term = a + ( 20 - 1 )d = a+19d = 4 + 19(5) = 99 .