Question

Find the sum for the 1st 20 terms in the sequence. 4, 7,10,13

Answer 1

First we begin by find the 20th terms, \[\huge a_{20}=a_{1}+(n-1)d\]

Answer 2

\[\huge a_{1}=4,\; n=20,\; d=3\]

Answer 3

Now we plug in our numbers:

Answer 4

I am sill lost

Answer 5

\[\huge a_{20}=61\]

Answer 6

Well, ill let this overzealous user explain it to you

Answer 7

Thank you for your help even though when I put in that answer it was wrong :(

Answer 8

\[\sum_{n=0}^{20}4+3n=4(21)+\frac{(3\times20)(3\times 21)}{2}=84+630=714\]

Answer 9

Notice here that \(\sum_{n=0}^k n=\frac{k(k+1)}{2}\).

Answer 10

Thank you [=

Answer 11

Or you can just add \(4+7+10+13+..+61\) :P