### Home > PC3 > Chapter 10 > Lesson 10.1.2 > Problem10-27

10-27.

Rewrite each of the following series using summation notation.

1. $\cos(1.2) + \cos(1.5) + \cos(1.8) + … + \cos(3.9)$

$\displaystyle \sum _ {k=0}^{\text{terms}-1}\text{cos(sequence)}$

sequence $= 1.2, 1.5, 1.8, …$
Write an expression for this sequence.

2. $1 − 3 + 5 − 7 + 9 − 11 + … + 201$

What expression would change the sign back and forth $(+ \text{ to } −)$?

$(−1)^n$
$n$ is an integer

What expression gives odd numbers?

$2n + 1$