Question

find the sum of 20 terms of an Ap 1,4,7,10....

Answer 1

Hope u like my process
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Formula to be used
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$= > s_{n} = \frac{n}{2} (2a + (n - 1)d) \\ \\ where \: \: s_{n} = sum \: \: of \: \: n \: \: terms \\ \\ \: \: \: \: \: \: \: \: \: \: \: \: \: \: a= first \: \: term \\ \\ \: \: \: \: \: \: \: \: \: \: \: \: \: \: d = difference \\ \\ \: \: \: \: \: \: \: \: \: \: \: \: \: \: n = number \: \: of \: \: terms$
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=> 1st term (a) = 1

=> difference (d ) = (4-1)=(7-4)=3

=> Sum of 20 terms

$= \frac{20}{2} ((2 \times 1) + (20 - 1)3) \\ \\ = 10(2 + (19 \times 3)) \\ \\ = 10(2 + 57) = 59 \times 10 \\ \\ = \bf \underline{ \: 590\: }$
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Hope this is ur required answer

Proud to help you

Answer 2

✌️HOPE it will HELP you☺️