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Find the sum : 25 + 28 + 31 +… + 100

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Find the sum : 25 + 28 + 31 +… + 100

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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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