Ontario 10 Academic (MPM2D)
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Perfect square trinomials
Lesson

Another type of binomial expression that can be factored very nicely are perfect squares. We've already come across some of these and know how to expand them:

$\left(a+b\right)^2=a^2+2ab+b^2$(a+b)2=a2+2ab+b2

$\left(a-b\right)^2=a^2-2ab+b^2$(ab)2=a22ab+b2

Notice a pattern in the expansion of $\left(a+b\right)^2$(a+b)2:

$a^2$a2 $+$+ $2ab$2ab $+$+ $b^2$b2
$\uparrow$   $\uparrow$   $\uparrow$
Square of the first term   Double the product of the two terms   Square of the second term

 

So if we have an expanded expression that fits this pattern, we can go backwards and factor it.

That is:

$a^2+2ab+b^2=\left(a+b\right)^2$a2+2ab+b2=(a+b)2

There is a very similar result with the differences of two squares, where we used the result of expansion to go backwards and factor.

 

Examples

Question 1

Which of the following are perfect square expansions?

A) $x^2+6x+9$x2+6x+9

Solution:

We can write $x^2+6x+9$x2+6x+9 as $\left(x\right)^2+2\left(3x\right)+\left(3\right)^2$(x)2+2(3x)+(3)2

We have the square of $x$x, the square of $3$3, and double the product of $x$x and $3$3. So this is a perfect square expression that can be factored.

 

B) $x^2-4x+16$x24x+16

Solution:

We can write $x^2-4x+16$x24x+16 as $\left(x\right)^2-4x+\left(-4\right)^2$(x)24x+(4)2

We have the square of $x$x and the square of $\left(-4\right)$(4). But the other term, $-4x$4x is not double the product of $x$x and $\left(-4\right)$(4). So this is not a perfect square expression.

 

C) $x^2-18x+81$x218x+81

Solution:

We can write $x^2-18x+81$x218x+81 as $\left(x\right)^2+2\left(-9x\right)+\left(-9\right)^2$(x)2+2(9x)+(9)2

We have the square of $x$x, the square of $\left(-9\right)$(9), and double the product of $x$x and $\left(-9\right)$(9). So this is a perfect square expression that can be factored.

 

D) $9+12x+4x^2$9+12x+4x2

Solution:

We can write $9+12x+4x^2$9+12x+4x2 as $\left(3\right)^2+2\left(3\times2x\right)+\left(2x\right)^2$(3)2+2(3×2x)+(2x)2

We have the square of $3$3, the square of $2x$2x, and double the product of $3$3 and $2x$2x. So this is a perfect square expression that can be factored.

 

Question 2

By expanding $\left(b+7\right)^2$(b+7)2, we want to determine the factoring for $b^2+14b+49$b2+14b+49.

  1. First expand $\left(b+7\right)^2$(b+7)2.

  2. Hence, factor $b^2+14b+49$b2+14b+49.

 
Question 3

Factor the expression $36y^2-12yz+z^2$36y212yz+z2

Think about how to attain $a$a and $b$b, and how to check if this really is a perfect square expression.

Do 

$\sqrt{36y^2}$36y2 $=$= $\sqrt{36}\sqrt{y^2}$36y2  
  $=$= $6y$6y  
$\sqrt{z^2}$z2 $=$= $z$z  
Therefore     $a$a is $6y$6y and our $b$b is $z$z.
$2\times a\times b$2×a×b $=$= $2\times6y\times z$2×6y×z  
  $=$= $12yz$12yz
which is the same as the middle term so this is a perfect square expression
So      
$36y^2-12yz+z^2$36y212yz+z2  $=$= $\left(6y-z\right)^2$(6yz)2  

 

Question 4

Factor $x^2-12x+36$x212x+36.

Question 5

Factor the following expression completely: $3p^2+12p+12$3p2+12p+12

Think about how to make this a perfect square expression by first factoring all the coefficients.

Do: We can see here that $3p^2$3p2 and $12$12 are not perfect squares! So how can we find $a$a and $b$b?

If we look at this another way we can see that the coefficients of all three terms have an GCF of $3$3, so let's factor that out first.

$3p^2+12p+12=3\left(p^2+4p+4\right)$3p2+12p+12=3(p2+4p+4)

Now the expression in the brackets is a perfect square expression where $\sqrt{p^2}=p$p2=p and $\sqrt{4}=2$4=2.

Therefore $3\left(p^2+4p+4\right)=3\left(p+2\right)^2$3(p2+4p+4)=3(p+2)2

 

Watch Out

Always see if you can factor expressions using the most basic method of $AB+AC=A\left(B+C\right)$AB+AC=A(B+C) first if you can. It'll make things a lot easier later on! 

 

Question 6

Factor $-18b^2-12ab-2a^2$18b212ab2a2.

Outcomes

10D.QR3.02

Factor polynomial expressions involving common factors, trinomials, and differences of squares, using a variety of tools and strategies

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