Completing the Square: Two Forms – Two Methods

Consider two expansions of (a+b)^2a^2+2ab+b^2 and a^2+b(2a+b).  These expressions are algebraically equivalent yet each contains an implicit point of view that leads to a different method for completing the square.

Let’s look at a specific problem.  Express y = x^2+6x+10 in the standard form y = a(x-h)^2+k.

Usual Method

Here a is 1 so we try to look at x^2+6x+10 as an expression like (x-h)^2 = x^2-2xh+h^2.  We already have x^2 so 6x = -2xh and h=-3 , h^2=9.  So x^2+6x+10 =x^2+6x+9+10-9 and we end up with y=(x+3)^2+1.  Note the usual trick of adding and subtracting 9.  Rather unaesthetic and hard for students to remember.

Another Method

Look at x^2+6x+10 as x^2-h(2x-h).  We have the x^2 already so 6x+10=-h(2x-h)h must be -3 so 6x+10=-(-3)(2x-(-3)=6x+9.  Oops.  We have 1 leftover (10-9) so y=(x+3)^2+1.   As described this method is nearly as awkward as the usual method.   This is because the way  the explanation was written obscures the steps. The “Another Method” uses the steps for finding the square root of any polynomial.  This method can be found in A College Algebra by G.A. Wentworth [1].

The basic idea is that we are trying to find \sqrt{N}, where N is some number or polynomial.  We know that \sqrt{N} = a+b and somehow we know a , the “larger” part.  We want to find b or at least refine our knowledge about bN = a^2+b(2a+b).  So we estimate b with \frac{N-a^2}{2a}, calculate N-(a^2+b(2a+b) and continue,  using a+b as our new a if the expression is not equal to zero.  This is the old-fashioned (and slow) way to find the square root of  a number by hand.  See for an example.

Here is the method applied to our example polynomial using the usual long division-type layout.This is a very clean procedure for completing the square.  The “remainder” appears naturally without the trick of adding and subtracting h^2.  It has the additional advantage of generalizing to finding the square root of any polynomial or number.

The algorithm is fun to use.  Try \sqrt{x^2+7x+10} and \sqrt{3x^2+5x+10} with first step a = \sqrt{3}x and compare to \sqrt{3x^2+5x+10} = \sqrt{3}\sqrt{x^2+\frac{5}{3}x+\frac{10}{3}}.

There is a comparable procedure for finding cube roots in Wentworth’s text.  I spent some time trying to solve quartic and cubic polynomial equations by taking square and cube roots but didn’t find any new cool  easy procedures.

Now to make a meta-point.  Different forms on an expression can give one different insights into the structure of a problem.  It is sometimes useful to look at a different version of an expression and construct a narrative that “explains” the form.

[1]  G.A. Wentworth, A College Algebra,Revised Edition, Ginn and Company, 1902.

About jrh794

I am a sixty-five year old math instructor at Southern Oregon University. I taught at the College of the Siskiyous in Weed California for twenty-six years. Prior to that I worked as a computer programmer, carpenter and in various other jobs. I graduated from Rice University in 1967 and have a MS in Operations Research from Stanford. In the past I have hand-built a stone house and taken long solo bicycle tours. Now I ride my mountain bike and play golf for recreation.
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2 Responses to Completing the Square: Two Forms – Two Methods

  1. G says:

    Showing a rectangle diagram as a visual interpretation really would help. Not enough instructors take advantage of that. Not enough textbooks show it. Completing the Square literally IS Completing the Square.

  2. Pingback: Mental Math – Square Roots | Math Thoughts

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