An important question from arithmetic progression chapter in which we have been asked to find which term of the AP 21, 18, 15,….is -81?
Book RS Aggarwal, Class 10, chapter 5A, question no 13
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Given AP is 21,18,15,....
First term =a=21
Common difference =d=18−21=−3
Last term =l=0
l=a+(n−1)d
0=21+(n−1)(−3)
0=21−3n+3
24−3n=0
or n=8
Answer: Zero is the 8th term.