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

In this Question the first and last term and common difference of the AP is given,

You have to find the

(i) Number of terms in the AP

(ii) Sum of the AP

This is the Question Number 29 Of Exercise 11 C of RS Aggarwal Solution

Deepak BoraNewbie

# The first and last terms of an AP are 4 and 81 respectively. If the common difference is 7, How many terms are there in the AP and what is their sum?

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a = 4l = 81d = 7l = a+ [n-1]d81 = 4 + [n-1] 7

77 = [n-1] 7

n-1 = 11

n = 12 = no. of terms

sum = n/2 [ a + l ]

sum = 12/2 [4 + 81 ]

sum = 6 x 85

sum = 510