Q5 of 46 Page 1

Find the sum of the first 25 terms of an A.P. whose nth term is given by tn = 2 – 3n.

∵, nth term is given by tn = 2 -3n

We shall find the first and last term of the A.P.


First put n = 1 in tn,


t1 = 2 – 3(1) = 2 – 3 = -1


Then put n= 25 in tn,


t25 = 2 – 3(25) = 2 – 75 = -73


t1 and t25 are first and last term of A.P respectively.


The sum of first 25 terms of this A.P is given by


Sum =


⇒ Sum =


⇒ Sum = -925


Hence, sum of the first 25 terms of the A.P is -925.


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