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 *n*th term.

Question Number 26 of Exercise 11 C of RS Aggarwal Solution

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

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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 .