Substitution and evaluating expressions

[[☃ video 1]]

##How to evaluate an expression with two variables

Let's say we want to evaluate the expression $10 + 2p - 3r$. Well, first we need to know the values of $p$ and $r$. For example, to evaluate the expression when $\blueD {p = 4}$ and $\goldD{r = 5}$, we just replace $\blueD p$ with $\blueD 4$ and $\goldD r$ with $\goldD 5$:

>$\phantom{=}10 + 2\blueD p - 3\goldD r$

>$=10 + 2\cdot\blueD 4 - 3 \cdot \goldD5$

>$=10 + 8 - 15$

>$=3$

So, the expression $10 + 2p - 3r$ equals $3$ when $p = 4$ and $r = 5$.

##Now, let's practice

[[☃ graded-group-set 1]]

[[☃ video 1]]

##Let's study another example.

Evaluate $\dfrac13a-1-\dfrac12b$ when $a=12$ and $b=6$.

$\begin{align}
&\phantom{=}\dfrac13\blueD a-1-\dfrac12\goldD b
\\
&= \dfrac13 \cdot \blueD {12}-1-\dfrac12 \cdot \goldD 6~~~~~~~~\text{Replace }\blueD{a} \text{ with } \blueD{12}\text{ and } \goldD b \text{ with } \goldD 6\text{.}
\\
&=4 - 1 - 3
\\
&=0
\end{align}$

##Now, let's practice

[[☃ graded-group-set 1]]