Question

find the sum of first 20 terms of an AP whoe first term is 3 and last term is 57

Answer 1

$S_n = \frac{n}{2} [T_1 + T_n] \\ \\ = \frac{20}{2} [3 + 57] \\ \\ = 10 \times 60 = 600$

600 is the answer.

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Answer 2

Here is your ans...☺️
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n=20 , an=57, a=3

sn=n/2 ( a+l)
=20/2(3+57)
=10(60)
=600
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