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What is the sum of the first 18th terms of an AP. If the 1st term is -20 and 18th term is 31?

What is the sum of the first 18th terms of an AP. If the 1st term is -20 and 18th term is 31? Correct Answer 99

GIVEN:

1st term is -20 and 18th term is 31

FORMULA USED:

an = a + (n - 1) × d

Sn = n/2(2a + (n - 1) × d)

CALCULATION:

a = - 20

And, a18 = 31

Sn = n/2(a + an)

⇒ S18 = 18/2(- 20 + 31)

⇒ S18 = 9 × 11 = 99

Thus the correct answer is 99.

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