### Home > INT3 > Chapter 10 > Lesson 10.1.3 > Problem10-43

10-43.

What are the sums of the arithmetic series below?

1. $5 + 10 + 15 + 20 + … + 400$

Find the sequence that describes the series.

$t\left(n\right) = 5n$

Find which term is $400$.

$\left(400\right) = 5n$
$80 = n$

Find the sum using the formula from the Math Notes box.

$S(80)=\frac{80(5+400)}{2}$

$16200$

1. $3 + 8 + 13 + 18 + … + 398$

See part (a).

$16040$

1. $80 + 74 + 68 + 62 + … + 14$

See part (a).