Q13 of 102 Page 410

If the sum of a certain number of terms of the AP 27, 24, 21, 18, …. is –30, find the last term.

To Find: Last term of the AP.


Let the number of terms be n.




⇒n[54 - 3n + 3] = - 60


⇒3n2 - 57n - 60 = 0



Either n = 20 or n = - 1 (n cannot be negative)


Therefore n = 20


Also,


, where l is the last term.



⇒ - 30 = 270 + 10l



⇒l = - 30


The last term is - 30.


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