Chapter 9 - Section 9.2 - The Dot Product - 9.2 Exercises - Page 646: 1

$a_1b_1+a_2b_2$ $scalar$

Step 1. Following the definition of a dot product, we fill in the first blank with $a_1b_1+a_2b_2$ Step 2. By definition, a dot product of two vectors is a real number or a scalar, not a vector, so we fill in the second blank with $scalar$