Answer :

∵, n^{th} term is given by t_{n} = 2 -3n

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

First put n = 1 in t_{n},

t_{1} = 2 – 3(1) = 2 – 3 = -1

Then put n= 25 in t_{n},

t_{25} = 2 – 3(25) = 2 – 75 = -73

t_{1} and t_{25} 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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