## Sunday, October 14, 2012

### Mathematical Practices

The Common Core State Standards list Standards for Mathematical Practice at each grade level.  These practices are

Mathematical Practices
1.     Make sense of problems and persevere in solving them.
2.     Reason abstractly and quantitatively.
3.     Construct viable arguments and critique the reasoning of others.
4.     Model with mathematics.
5.     Use appropriate tools strategically.
6.     Attend to precision.
7.     Look for and make use of structure.
8.     Look for and express regularity in repeated reasoning.

These same eight expectations are listed in the descriptions for every grade level and for every advanced (a.k.a. high school) course.  But carrying out the Mathematical Practices will look different at different grade levels.

The practices that may require the most clarification are probably #4 (Model with mathematics), #7 (Look for and make use of structure) and #8 (Look for and express regularity in repeated reasoning).

Modeling with mathematics typically involves using and perhaps even creating mathematical objects (such as algebraic expressions, equations, inequalities, graphs, etc.) to capture key aspects of a situation to be explored.  A kindergartner might use 2+3 to represent the number of people involved if two people are joined by 3 more; a sixth grader might describe  a relationship between the numbers of tables to chairs in a room by the ratio 1:4; an Algebra I student might use the expression 10x to represent the value (in cents) of x dimes.

But not all word problems involve mathematical modeling.  It is not mathematical modeling to use a contrived algebraic expression such as a quadratic expression obtained by curve-fitting bi-variate data without any plausible a priori reason for believing that the two variables should be related quadratically.

Mathematical Practice #7, seeking  and using structure, is key to both pure and applied mathematics.  A first-grader begins to recognize that an addition fact such as 2+3=5 carries with it a family of related arithmetic facts, e.g., 3+2=5, 5-2=3, 5-3=2, etc. ; a seventh-grader can see that because a+0.05a = 1.05a, increasing a quantity by 5% is equivalent to scaling the quantity by 1.05; a geometry student recognizes and introduces structure by adding an auxiliary line to a geometric diagram.  Mathematical Practice #7 is definitely not about memorizing or plugging into formulas--both practices,  when applied inappropriately, can allow students to ignore the underlying structure .

I personally need further explanation of Mathematical Practice #8.  Here is how it's first described in the CCSS:
Mathematically proficient students notice if calculations are repeated, and look both for general methods and for shortcuts. Upper elementary students might notice when dividing 25 by 11 that they are repeating the same calculations over and over again, and conclude they have a repeating decimal. By paying attention to the calculation of slope as they repeatedly check whether points are on the line through (1, 2) with slope 3, middle school students might abstract the equation $$\frac {y – 2}{x – 1} = 3$$ . Noticing the regularity in the way terms cancel when expanding (x – 1)(x + 1), (x – 1)(x^2 + x + 1), and (x – 1)(x^3 + x^2 + x + 1) might lead them to the general formula for the sum of a geometric series. As they work to solve a problem, mathematically proficient students maintain oversight of the process, while attending to the details. They continually evaluate the reasonableness of their intermediate results.

The phrase "repeated reasoning" presumably refers to the "R" in Guershon Harel's DNR. Some of Harel's work is listed in the mathematics CCSS references, but there do not appear to be any direct attributions cited.