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# Single Variable Expressions

## Use symbols and operations to understand and define variables.

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Practice Single Variable Expressions

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Dog Diets

Do you have a pet dog, or have you ever wanted one? Feeding your canine companion is obviously an important thing to consider. But what if you didn’t know how many calories to feed the dog and only knew the dog’s weight? Could you use an expression to figure how many calories the dog should eat? What values would you need to know?

#### Why It Matters

Veterinarians have figured out a simple way to calculate how many calories are needed to feed dogs of different weights and sizes. If you take the weight of your dog in kilograms, multiply it by 30, and add 70, you can get your dog’s resting energy requirement (RER) in calories:

\begin{align*}30(\text{dog's weight})+70=\text{RER}\end{align*}

Notice that you have a variable expression to evaluate; you only need to know the dog’s weight in order to calculate the number of calories needed. Medium-sized dogs like the Boxer above weigh an average of 28 kilograms. How many calories would the RER of such a dog be?

To find out, substitute 28 for the dog’s weight in the expression above:

\begin{align*}30(28)+70=910\end{align*}

Thus, a 28-kilogram Boxer would have an RER of 910 calories.

It’s important to note, however, that the RER, or resting energy requirement, does not take the dog’s activity into consideration. You can see how activity level factors into canine diets here:
http://youtu.be/w-Xj6a78E1k

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