Suppose you have the variable expression $$\frac{3m-2}{8}$$ This expressions means "I'm thinking of a number that is multiplied by 3, then subtracted by 2, and finally divided by 8." Evaluating an expression means replacing the variable with an actual number. For example, if m = 6, then $$\frac{3m-2}{8}=\frac{3\cdot~6-2}{8}=\frac{18-2}{8}=\frac{16}{8}=2$$ |
Example 1
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We don't know the number of marbles in each bag, so all we can say is that she bought 2x + 3 marbles.
If we are told there are 42 marbles in each of Martha's bags, then we can substitute 42 for the x in the variable expression.
2x + 3 ------> 2(42) + 3 = 84 + 3 = 87 marbles
Martha bought 87 marbles.
Example 2 Evaluate $2x+3$ if x = 17. |
Solution: Replace the variable with 17 and then simplify. |
Example 3
Jenny was filling bottles from a tank of water. When she had filled four bottles with water, 1 cup of water was left in the tank. How much water is in the tank? |
The variable expression for this situation is 4c + 1, where c represents the number of cups in each bottle.
If c = 6, how many cups of water are in the tank?
Example 4
If a = 3, b = 6, and c = 4 evaluate each of the following expressions.
$\large~ab+c$ |
$\large~\frac{bc}{2}$ |
$\large~\frac{a(b+4)}{c+1}$ |
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Question 1 Evaluate $4x-6$ for $x=4$. |
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Question 2 Evaluate $6x+2x$ for $x=3$. |
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Question 3 Evaluate $\frac{39}{x}+5y$ for $x=3\mbox{ and }y=5$. |
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