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Deepak Bora
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The sum of n term of an AP is [ 5n^2/ 2 + 3n/2 ]. Find its 20th term.

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This is the Important question based on Arithmetic progression Chapter of R.S Aggarwal book for ICSE & CBSE Board.
Here the sum of n term of an AP is given. You have to find its given nth term.
Question Number 26 of Exercise 11 C of RS Aggarwal Solution

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  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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