Q. 135.0( 2 Votes )

Find the sum : 25 + 28 + 31 +… + 100

Answer :

Here, a = 25, d = 28 – 25 = 3 and an = 100


We know that,


an = a + (n – 1)d


100 = 25 + (n – 1)3


75 = (n – 1)3


25 = n – 1


26 = n


Now,




S26 = 13[50 + 25 × 3]


S26 = 13[50 + 75]


S26 = 13 × 125


S26 = 1625


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