This is the Important question based on Arithmetic progression Chapter of R.S Aggarwal book for ICSE & CBSE Board.

Here the *n*th term of AP is given

you have to find the

(i) Sum of Given first n terms of the AP

(ii)Sum of n terms of the AP

Question Number 23 of Exercise 11 C of RS Aggarwal Solution

Deepak BoraNewbie

# If the nth term of an AP is (4n+1) find the sum of the first 15 terms of the AP. Also find the sum of its n terms.

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The a_n term of the AP is given

=(4n+1)

so that a_1 is :

a_1=4(1)+1

a_1=5

The Sum of first 15 terms of the AP is :

S_15=n/2[a_1+a_15]

S_15=15/2[5+4(15)+1]

S_15=15/2[5+61]

S_15=15/2×66

S_15=495

the sum of first 15 term is 495

The sum of n Terms of the AP is:

S_n=n/2[a_1+a_n]

S_n=n/2[5+4n+1]

S_n=n/2[6+4n]

S_n=n[3+2n]

the sum of n terms of the AP is n[3+2n]