Writing Expressions
Task
Write an expression for the sequence of operations.
- Add 3 to x, subtract the result from 1, then double what you have.
- Add 3 to x, double what you have, then subtract 1 from the result.
IM Commentary
The instructions for the two expressions sound very similar, however, the order in which the different operations are performed and the exact wording make a big difference in the final expression. Students have to pay close attention to the wording: “subtract the result from 1” and “subtract 1 from the result” are very different.
Solution
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This problem can be done step-by-step. We first add 3 to x:
x+3.Then we subtract the result that we just got from 1:
1-(x+3).We then double, meaning we multiply this entire expression by 2:
2(1-(x+3)).If we choose to simplify this expression, we use the distributive, commutative and associative properties in the following way:
\begin{alignat}{2} 2(1-(x+3)) &= 2(1-x-3) &\qquad &\text{distribute the -1} \\ &= 2(-x - 2) &\qquad &\text{subtracting 3 from 1} \\ &= -2x - 4 &\qquad &\text{distribute the 2} \\ \end{alignat} -
Again, we add 3 to x:
x+3This time, next we double, meaning multiplying this expression by 2:
2(x + 3).Then we subtract 1 from the result and we have:
2(x+3)−1.If we choose to simplify this expression, we use the distributive and associative properties in the following way:
\begin{alignat}{2} 2(x+3)-1 &= (2x+6)-1 &\qquad &\text{distribute the 2} \\ &=2x + 5 &\qquad &\text{subtracting 1 from 6} \end{alignat}Notice that the final expressions are very different, even though the instructions sounded very similar.
Writing Expressions
Write an expression for the sequence of operations.
- Add 3 to x, subtract the result from 1, then double what you have.
- Add 3 to x, double what you have, then subtract 1 from the result.