Mathematics · Arithmetic Progressions · NCERT Examples
Example 4
A complete, independently verified solution for NCERT Example 4.
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Mathematics · Chapter 5 · NCERT Examples
Verified NCERT questions and solutionsSolutions are checked independently and follow the supplied NCERT chapter edition.
NCERT Example
Example 4
Which term of 21,18,15,… is −81? Is any term 0?
Solution
Board-exam working:
Here a=21 and d=18−21=−3.
For −81: −81=21+(n−1)(−3)=21−3n+3=24−3n.
Thus 3n=105 and n=35.
Check: a₃₅=21+34(−3)=21−102=−81.
For 0: 0=21+(n−1)(−3)=24−3n.
Thus 3n=24 and n=8.
Check: a₈=21+7(−3)=0.
Final answer
−81 is the 35th term; 0 is the 8th term.