Add/Subtract/Multiply/Divide Expressions

Divad2017

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May 6, 2008
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As a parent, I am confused as to how to write expressions. We are using the McGraw Hill Mathematics book (2002) for 5th grade. I understand the order of operations (PEMDAS) but am very confused how to evaluate the expression for value given. Example: 125 - x + 13 for x = 43 or 200 + 32 X a for a =20. Can you help me describe this to my son in easier terms and how to do it? How is the work done to arrive at the answer? Thanks for anything you can do.
 

stapel

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masters

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The lesson is teaching "substitution" of a value in place of a variable and simplifying the expression after substitution.

125 - x + 13 is your original expression.

For x, substitute 43:

125 - (43) + 13

And simplify: 125 - 43 + 13 = 95

Perform similar steps in your second example to arrive at 200 + 32 times 20 = 840
 

Loren

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125 - x + 13 for x = 43

Would it help to physically make the substitution? I'm thinking you could write "x=43" on a piece of paper then carefully cut out just the 43. Put a dab of stickum on the back of that chard of paper and stick it right on top of the x in the expression so that it now reads 125 - 43 + 13.
I might suggest that at some later time he will be introduced to negative numbers in which case you always want to include the sign with the number by enclosing in parenthesis. In other words, if x=-43, the expression would become 125 - (-43) + 13. But, save that for the future.
 
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