This is the series 4+ 7 + 10 + 13

In other words it is

Consider the bars and how the represent the numbers in the sequence, and how the total sum of all of the bars has a certain area.

Well, clone this series of terms, and combine them to make a rectangle. This will happen for ALL arithmetic sequences. The area of a rectangle is given by A= width x height, so in this case 4(4+13) OR n(a1 + an )

But, we are only interested in half of this rectangle so divide by two, and we get.

4

1

13x

x

n

x

nn

aana

1

1

2

)(

^

is a sequence^