### Home > PC > Chapter 7 > Lesson 7.3.1 > Problem7-95

7-95.

Find the following sums.

1. $3 + 6 + 9 + … + 90$

Add the first and last terms.
Divide by 2.
Multiply by the number of terms.

$\frac{30(3+90)}{2}=1395$

1. $20 + 22 + 24 + … + 100$

How many terms are there?

$41$

1. $\displaystyle \sum _ { n = 1 } ^ { 41 } 2 n + 18$

Add the first and last terms.
Divide by 2.
Multiply by the number of terms.

1. $\displaystyle \sum _{ k = 8 } ^ { 53 } 5 k - 3$

How many terms are there? That is, if the sum started with k = 1, what would it end with?

$46$