Question

In an ap the sum of first n terms is 5n²/2 + 3n/2.Fine it's 20th term

Answer 1

Sum of n terms =[ 5n²/2 + 3n/2 ]

Sum of 1 term = 5*1/2 + 3(1)/2 = 8/2 = 4 .

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 .

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