Question

find the sum of 20 terms of the series whose rth term is 3r+1​

Answer 1

So for 1 : 3(1)+1=4

So for 2 : 3(2)+1=7

This is going to continue until 20 : 3(20)+1 = 61

From 1 and 2, we see there is a general increase in the results by 3

So the sum of the 20 terms:

4+7+10+13+16+19+22+25+28+31+34+37+40+43+46+49+52+55+58+61

= 650

So the sum of the 20 terms is 650 .

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