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Factor: r 2 3 r 40 .

( r + 5 ) ( r 8 )

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Factor: s 2 3 s 10 .

( s + 2 ) ( s 5 )

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Some trinomials are prime. The only way to be certain a trinomial is prime is to list all the possibilities and show that none of them work.

Factor: y 2 6 y + 15 .

Solution

y 2 6 y + 15 Factors will be two binomials with first ( y ) ( y ) terms y .

Factors of 15 Sum of factors
−1 , −15 −1 + ( −15 ) = −16
−3 , −5 −3 + ( −5 ) = −8

As shown in the table, none of the factors add to −6 ; therefore, the expression is prime.

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Factor: m 2 + 4 m + 18 .

prime

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Factor: n 2 10 n + 12 .

prime

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Factor: 2 x + x 2 48 .

Solution

2 x + x 2 48 First we put the terms in decreasing degree order. x 2 + 2 x 48 Factors will be two binomials with first terms x . ( x ) ( x )

As shown in the table, you can use −6 , 8 as the last terms of the binomials.

( x 6 ) ( x + 8 )
Factors of −48 Sum of factors
−1 , 48 −1 + 48 = 47
−2 , 24
−3 , 16
−4 , 12
−6 , 8
−2 + 24 = 22
−3 + 16 = 13
−4 + 12 = 8
−6 + 8 = 2

Check.

( x 6 ) ( x + 8 )

x 2 6 q + 8 q 48

x 2 + 2 x 48

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Factor: 9 m + m 2 + 18 .

( m + 3 ) ( m + 6 )

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Factor: −7 n + 12 + n 2 .

( n 3 ) ( n 4 )

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Let’s summarize the method we just developed to factor trinomials of the form x 2 + b x + c .

Factor trinomials.

When we factor a trinomial, we look at the signs of its terms first to determine the signs of the binomial factors.

x 2 + b x + c ( x + m ) ( x + n )

When c is positive, m and n have the same sign.

b positive b negative m , n positive m , n negative x 2 + 5 x + 6 x 2 6 x + 8 ( x + 2 ) ( x + 3 ) ( x 4 ) ( x 2 ) same signs same signs

When c is negative, m and n have opposite signs.

x 2 + x 12 x 2 2 x 15 ( x + 4 ) ( x 3 ) ( x 5 ) ( x + 3 ) opposite signs opposite signs

Notice that, in the case when m and n have opposite signs, the sign of the one with the larger absolute value matches the sign of b .

Factor trinomials of the form x 2 + bxy + cy 2

Sometimes you’ll need to factor trinomials of the form x 2 + b x y + c y 2 with two variables, such as x 2 + 12 x y + 36 y 2 . The first term, x 2 , is the product of the first terms of the binomial factors, x · x . The y 2 in the last term means that the second terms of the binomial factors must each contain y . To get the coefficients b and c , you use the same process summarized in the previous objective.

Factor: x 2 + 12 x y + 36 y 2 .

Solution

x 2 + 12 x y + 36 y 2 Note that the first terms are x , last terms contain y . ( x _ y ) ( x _ y )

Find the numbers that multiply to 36 and add to 12.

Factors of 36 Sum of factors
1, 36 1 + 36 = 37
2, 18 2 + 18 = 20
3, 12 3 + 12 = 15
4, 9 4 + 9 = 13
6, 6 6 + 6 = 12 *

Use 6 and 6 as the coefficients of the last terms. ( x + 6 y ) ( x + 6 y ) Check your answer. ( x + 6 y ) ( x + 6 y ) x 2 + 6 x y + 6 x y + 36 y 2 x 2 + 12 x y + 36 y 2

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Factor: u 2 + 11 u v + 28 v 2 .

( u + 4 v ) ( u + 7 v )

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Factor: x 2 + 13 x y + 42 y 2 .

( x + 6 y ) ( x + 7 y )

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Factor: r 2 8 r x 9 s 2 .

Solution

We need r in the first term of each binomial and s in the second term. The last term of the trinomial is negative, so the factors must have opposite signs.

r 2 8 r x 9 s 2 Note that the first terms are r , last terms contain s . ( r _ s ) ( r _ s )

Find the numbers that multiply to −9 and add to −8 .

Factors of −9 Sum of factors
1 , −9 1 + ( −9 ) = −8 *
−1 , 9 −1 + 9 = 8
3 , −3 3 + ( −3 ) = 0

Use 1 , −9 as coefficients of the last terms. ( r + s ) ( r 9 s ) Check your answer. ( r 9 s ) ( r + s ) r 2 + r s 9 r s 9 s 2 r 2 8 r s 9 s 2

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Factor: a 2 11 a b + 10 b 2 .

( a b ) ( a 10 b )

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Factor: m 2 13 m n + 12 n 2 .

( m n ) ( m 12 n )

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Factor: u 2 9 u v 12 v 2 .

Solution

We need u in the first term of each binomial and v in the second term. The last term of the trinomial is negative, so the factors must have opposite signs.

u 2 9 u v 12 v 2 Note that the first terms are u , last terms contain v . ( u _ v ) ( u _ v )

Find the numbers that multiply to −12 and add to −9 .

Factors of −12 Sum of factors
1 , −12 1 + ( −12 ) = −11
−1 , 12 −1 + 12 = 11
2 , −6 2 + ( −6 ) = −4
−2 , 6 −2 + 6 = 4
3 , −4 3 + ( −4 ) = −1
−3 , 4 −3 + 4 = 1

Note there are no factor pairs that give us −9 as a sum. The trinomial is prime.

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Factor: x 2 7 x y 10 y 2 .

prime

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Factor: p 2 + 15 p q + 20 q 2 .

prime

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Key concepts

  • Factor trinomials of the form x 2 + b x + c
    1. Write the factors as two binomials with first terms x : ( x ) ( x ) .
    2. Find two numbers m and n that
      Multiply to c , m · n = c
      Add to b , m + n = b
    3. Use m and n as the last terms of the factors: ( x + m ) ( x + n ) .
    4. Check by multiplying the factors.

Practice makes perfect

Factor Trinomials of the Form x 2 + b x + c

In the following exercises, factor each trinomial of the form x 2 + b x + c .

x 2 + 4 x + 3

( x + 1 ) ( x + 3 )

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m 2 + 12 m + 11

( m + 1 ) ( m + 11 )

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a 2 + 9 a + 20

( a + 4 ) ( a + 5 )

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p 2 + 11 p + 30

( p + 5 ) ( p + 6 )

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n 2 + 19 n + 48

( n + 3 ) ( n + 16 )

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a 2 + 25 a + 100

( a + 5 ) ( a + 20 )

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x 2 8 x + 12

( x 2 ) ( x 6 )

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y 2 18 x + 45

( y 3 ) ( y 15 )

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x 2 8 x + 7

( x 1 ) ( x 7 )

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p 2 + 5 p 6

( p 1 ) ( p + 6 )

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y 2 6 y 7

( y + 1 ) ( y 7 )

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x 2 x 12

( x 4 ) ( x + 1 ) ( x 4 ) ( x + 3 )

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a 2 3 a 28

( a 7 ) ( a + 4 )

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w 2 5 w 36

( w 9 ) ( w + 4 )

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8 6 x + x 2

( x 4 ) ( x 2 )

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x 2 12 11 x

( x 12 ) ( x + 1 )

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Factor Trinomials of the Form x 2 + b x y + c y 2

In the following exercises, factor each trinomial of the form x 2 + b x y + c y 2 .

p 2 + 3 p q + 2 q 2

( p + q ) ( p + 2 q )

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r 2 + 15 r s + 36 s 2

( r + 3 s ) ( r + 12 s )

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m 2 12 m n + 20 n 2

( m 2 n ) ( m 10 n )

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x 2 2 x y 80 y 2

( x + 8 y ) ( x 10 y )

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p 2 8 p q 65 q 2

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m 2 64 m n 65 n 2

( m + n ) ( m 65 n )

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p 2 2 p q 35 q 2

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a 2 + 5 a b 24 b 2

( a + 8 b ) ( a 3 b )

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x 2 3 x y 14 y 2

prime

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u 2 8 u v 24 v 2

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m 2 5 m n + 30 n 2

prime

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Mixed Practice

In the following exercises, factor each expression.

u 2 12 u + 36

( u 6 ) ( u 6 )

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x 2 14 x 32

( x + 2 ) ( x 16 )

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r 2 20 r s + 64 s 2

( r 4 s ) ( r 16 s )

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k 2 + 34 k + 120

( k + 4 ) ( k + 30 )

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m 2 + m n 56 n 2

( m + 8 n ) ( m 7 n )

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q 2 29 q r 96 r 2

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u 2 17 u v + 30 v 2

( u 15 v ) ( u 2 v )

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c 2 8 c d + 26 d 2

prime

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Everyday math

Consecutive integers Deirdre is thinking of two consecutive integers whose product is 56. The trinomial x 2 + x 56 describes how these numbers are related. Factor the trinomial.

( x + 8 ) ( x 7 )

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Consecutive integers Deshawn is thinking of two consecutive integers whose product is 182. The trinomial x 2 + x 182 describes how these numbers are related. Factor the trinomial.

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Writing exercises

Many trinomials of the form x 2 + b x + c factor into the product of two binomials ( x + m ) ( x + n ) . Explain how you find the values of m and n .

Answers may vary

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How do you determine whether to use plus or minus signs in the binomial factors of a trinomial of the form x 2 + b x + c where b and c may be positive or negative numbers?

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Will factored x 2 x 20 as ( x + 5 ) ( x 4 ) . Bill factored it as ( x + 4 ) ( x 5 ) . Phil factored it as ( x 5 ) ( x 4 ) . Who is correct? Explain why the other two are wrong.

Answers may vary

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Look at [link] , where we factored y 2 + 17 y + 60 . We made a table listing all pairs of factors of 60 and their sums. Do you find this kind of table helpful? Why or why not?

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Self check

After completing the exercises, use this checklist to evaluate your mastery of the objectives of this section.

This table has the following statements all to be preceded by “I can…”. The first is “factor trinomials of the form x ^ 2 +b x + c”. The second is “factor trinomials of the form x^2 + b x y + c y ^ 2”. In the columns beside these statements are the headers, “confidently”, “with some help”, and “no-I don’t get it!”.

After reviewing this checklist, what will you do to become confident for all goals?

Questions & Answers

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Source:  OpenStax, Elementary algebra. OpenStax CNX. Jan 18, 2017 Download for free at http://cnx.org/content/col12116/1.2
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