Understanding Algebra

James W. Brennan
 

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Factoring a Quadratic Trinomial by Grouping

Another method for factoring these kinds of quadratic trinomials is called factoring by grouping. Factoring by grouping can be a bit more tedious, and is often not worth the trouble if you can find the correct factors by some quick trial and error. However, it works quite well when the factors are not immediately obvious, such as when you have a very large number of candidate factors. When this happens, the trial and error method becomes very tedious

Factoring by grouping is best demonstrated with a few examples.

 

Example: 

Given:

5x2 + 11x + 2

Find the product ac:

(5)(2) = 10

Think of two factors of 10 that add up to 11:

1 and 10

Write the 11x as the sum of 1x and 10x:

5x2 + 1x + 10x + 2

Group the two pairs of terms:

(5x2 + 1x) + (10x + 2)

Remove common factors from each group:

x(5x + 1) + 2(5x + 1)

Notice that the two quantities in parentheses are now identical. That means we can factor out a common factor of (5x + 1):

(5x + 1)(x + 2)

 

Example: 

Given:

4x2 + 7x – 15

Find the product ac:

(4)(-15) = -60

Think of two factors of -60 that add up to 7:

-5 and 12

Write the 7x as the sum of -5x and 12x:

4x2 – 5x + 12x – 15

Group the two pairs of terms:

(4x2 – 5x) + (12x – 15)

Remove common factors from each group:

x(4x – 5) + 3(4x – 5)

Notice that the two quantities in parentheses are now identical. That means we can factor out a common factor of (4x - 5):

(4x – 5)(x + 3)

 

The Procedure

Given a general quadratic trinomial

ax2 + bx + c

1. Find the product ac.

2. Find two numbers h and k such that

hk = ac

(h and k are factors of the product of the coefficient of x2 and the constant term)

AND

h + k = b

(h and k add to give the coefficient of x)

3. Rewrite the quadratic as

ax2 + hx + kx + c

4. Group the two pairs of terms that have common factors and simplify.

(ax2 + hx) + (kx + c)

x(ax + h) + (kx + c)

(note: because of the way you chose h and k, you will be able to factor a constant out of the second parentheses, leaving you with two identical expressions in parentheses as in the examples).

·        Remember that this won’t work for all quadratic trinomials, because not all quadratic trinomials can be factored into products of binomials with integer coefficients. If you have a non-factorable trinomial, you will not be able to do step 2 above.

Why this works

Suppose the quadratic trinomial in question came from multiplying two arbitrary binomials:

(px + n)(qx + m)

If we multiply this out we will get

pqx2 + pmx + qnx + nm

or

pqx2 + (pm + qn)x + nm

Notice that the coefficient of x consists of a sum of two terms, pm and qn. These are the two numbers we called h and k above.

pm = h

qn = k

Now we see that the two numbers h and k add up to the coefficient of x, which we called b:

h + k = b

Obviously they are factors of their own product pmqn, but we notice that pq = a, and mn = c, so

(pm)(qn) = (pq)(nm)

which is equivalent to

hk = ac

 

 

      

 


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James W. Brennan