Question

find the sum of first 20 terms of series 4+ 7+ 10+ .........

Answer 1

Hi friend,

Here is your answer,

Sn = 20/2[2*4+(20-1)3]
Sn = 10[8 + 57]
Sn = 10[65]
Sn = 650

HOPE THIS HELPED YOU !!
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Answer 2

Here n=20, and d=3 with a=4.
Using formula Sn=n/2(2a+(n-1)d) we get
20/2(2x4+19x3) = 10x65 = 650