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

term number

Which term of 21,18,15,… is −81? Is any term 0?

Board-exam working:

a_n=a+(n-1)d

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.

−81 is the 35th term; 0 is the 8th term.