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Question
Find the sum of first 20 terms of an A.P. whose first term is 3 and the last term is 57.
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Solution
Here,
First term, a = 3
Last term, l = 57
n = 20
`S = n/2 (a + I)`
= `20/2 (3 + 57)`
= 10 × 60
= 600
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