Math Tip of the Week – Order of Operations

Order of Operations (in Pre-Algebra) really are a lot of fun – like a puzzle!  And remember that the calculator does them automatically!

I like to use the mnemonic “Please Excuse MDear Aunt Sally” or PEMDAS to remember the order.   The order is Parentheses,Exponents, Multiplication and Division (either one, in order from left to right), and then Addition and Subtraction (either one, in order from left to right) – PEMDAS.

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My Dear Aunt Sally

PEMDAS stands for:

 PLEASE EXCUSE MY DEAR AUNT SALLY Parentheses Exponents Multiplication Division Addition Subtraction Left to right, perform either multiplication OR division Left to right, perform either addition OR subtraction

So you’ll do the math left to right, like you’re reading a book, but you must pay careful attention and perform some operations before others.  Here are some examples:

 Order of Operation Example Explanation $$\begin{array}{c}\color{#800000}{{5+8\div 2}}\\5+4\\9\end{array}$$ Since we have to perform multiplication and division before addition and subtraction, we have to first divide 8 by 2, which is 4, and then add it to 5, which is 9. $$\begin{array}{c}\color{#800000}{{4-{{{\left( {5-3} \right)}}^{2}}\times 6}}\\4-{{\left( 2 \right)}^{2}}\times 6\\4-4\times 6\\4-24\\-20\end{array}$$ We have to first subtract the 3 from the 5, since they are in the parentheses; this equals 2. Then we have to square 2, since we have to perform exponents next; we get 4. Then we have to multiply 4 by 6 since we need to do multiplication before subtraction. Then we have to subtract 24 from 4 to get  –20. $$\begin{array}{c}\color{#800000}{{10\div 2\times 6-3}}\\5\times 6-3\\30-3\\27\end{array}$$ This one is a little tricky. Since we have to perform multiplication or division from left to right, we need to divide 10 by 2 first to get 5. Then we have to multiply the 5 by 6 to get 30, and finally subtract the 3 from 30 to get 27. $$\displaystyle \begin{array}{c}\color{#800000}{{5+{{{\left[ {-1\,\left( {-4-2} \right)} \right]}}^{2}}}}\\5+{{\left[ {-1\left( {-6} \right)} \right]}^{2}}\\5+{{[6]}^{2}}\\5+36\\41\end{array}$$ Sometimes we have two sets of parentheses in problems, where we use parentheses inside brackets. We have to perform the operations from the inside out, so we first have to subtract the –2 from the –4 to get –6. Then we have to multiply it by –1 to get 6. Then we have to square the 6 to get 36, and add it to the 5 to get 41. $$\displaystyle \begin{array}{c}\color{#800000}{{4-\left| {8-10} \right|\cdot {{3}^{2}}-1}}\\4-\left| {-2} \right|\cdot {{3}^{2}}-1\\4-\left( 2 \right)\cdot \left( 9 \right)-1\\4-18-1\\-15\end{array}$$ Treat the absolute value sign as a parenthesis, so do that math first. Note that the absolute value of –2 is 2. Then we square the 3 to get 9, do the multiplication, and finally the subtraction. $$\displaystyle \frac{{100-25}}{{4+3\times 2}}+4\times 10$$ $$\displaystyle \frac{{\left( {100-25} \right)}}{{\left( {4+3\times 2} \right)}}+40$$ $$\displaystyle \frac{{75}}{{10}}+40$$ $$\displaystyle \begin{array}{c}7.5+40\\47.5\end{array}$$ Treat the operations in the numerator and denominator as having parentheses; we’ll then do the multiplication and division operations before the addition and subtraction.

Happy Mathing!

Lisa

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