An important question from ML Aggarwal(Avichal Publication)
Find the number of terms of the A.P; 24,21,18,... must be taken such that their sum is 78.
Sir please give me a detailed solution of this question as it is from Arithmetic Progression (chapter 9)
Sn=2n[2a+(n−1)d]
Given,
a=24,d=21−24=−3,Sn=78
78=2n[2(24)+(n−1)(−3)]
156=n[48−3n+3]
3n2−51n+156=0
n2−17n+52=0
(n−13)(n−4)=0
∴n=4,13