This is the Important question based on Arithmetic progression Chapter of R.S Aggarwal book for ICSE & CBSE Board.
Here you have to find the
(i) Number of terms so that their sum is Given
(ii)Explain the double answer
Question Number 21 Of Exercise 11 C of RS Aggarwal Solution.
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Find the number of terms of the AP 64,60,56,……so that their sum is 544. Explain the double answer.
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The given AP is 64,60,56,……………….
We know that,
First term (a) = 64
Common difference =
= 60-64 = -4
Given sum of AP (
) = 544
By using formula:-
⇒
⇒%3D0)
⇒
⇒
⇒-17(n-16)%3D0)
⇒%20(n-17)%3D0)
⇒
and 
⇒ n = 16 and n = 17
so, for the values n=16 and n=17 the sum of the given A.P. is 544.