1. Factoring Quadratic Trinomials 5/6/01
Objective: you will factor quadratic polynomials into a pair of binomials

2. OUTLINE
Put in descending order of exponents like ax²+bx+c and multiply a*c
Find two numbers whose product is (a*c), BUT whose sum is ‘b’
Rewrite ax²+ [larger factor]x + [smaller factor]x +c ( larger factor takes the same sign as ‘b’)
Factor the first pair of terms and factor the second pair of terms(by dividing out the common factor for each pair and if the 3rd term is negative, then factor it out also)
Factor out the common binomial factor
Write as a pair of binomials

3. Example 1 Factor x²+6x+8 Order and multiply a*c
Find factors of (a*c), but their sum is ‘b’
Write the new polynomial
Double Factor
Factor the binomial
Write a pair of binomials
Factors of 8 Sum of 6?
1,8 9
2,4 6
-1,-8 -9
-2,-4 -6
x²+4x+2x+8
x(x+4) +2(x+4)
(x+4)(x+2)

4. Example 2 Factor x²–4x -21 x² -7x+3x – 21 x(x-7) +3(x-7) (x-7) (x+3)
Factors of –21,Sum of –4? 1,
3,-7 -4
-1,21 20
-3,7 4
x² -7x+3x – 21
x(x-7) +3(x-7)
(x-7) (x+3)
Order and multiply a*c
Find factors of (a*c), but their sum is ‘b’
Rewrite the new polynomial
Double Factor
Factor the binomial
Write a pair of binomials

5. Example 3 Factor y²+7y-30 y²+ 10y –3y –30 y(y+10) –3(y+10)
Order and multiply a*c
Find factors of (a*c), but their sum is ‘b’
Rewrite the new polynomial
Double Factor
Factor the binomial
Write a pair of binomials
Factor –30 Sum of 7? 1, 2, 3,
5,-6 -1
-1,30 29
-2,15 13
-3, 10 7
-5,6 1
y²+ 10y –3y –30
y(y+10) –3(y+10)
(y+10) (y-3)

6. Example 4 Factor 2x² –13x +15 Order and multiply a*c
Find factors of (a*c), but their sum is ‘b’
Rewrite the new polynomial
Double Factor
Factor the binomial
Write a pair of binomials
Factors of 30 Sum of –13?
1,30 31
2,15 17
3,10 13
5,6 11
-1-,
-2,
-3,
-5,
2x² -10x -3x +15
2x(x -5) -3(x -5)
(x -5)(2x –3)

7. Example 5 Factor 15r² +44r +21 15r² +35r +9r +21 5r(3r +7) +3(3r +7)
Order and multiply a*c
Find factors of (a*c), but their sum is ‘b’
Rewrite the new polynomial
Double Factor
Factor the binomial
Write a pair of binomials
Factors of 315 Sum of 44? 1, 3, 5, 7, 9,
15r² +35r +9r +21
5r(3r +7) +3(3r +7)
(3r +7)(5r +3)

8. Check your understanding
Check your understanding (click to check each step) 6) Factor x² +2x – )Factor 3y² -22y –16
x² +2x –80 (no peeking)
Factors of –80 Sum of 2?
-1,
-2,
-4,
-8, 10 2
x² +10x –8x –80
x(x +10) –8(x +10)
(x +10)(x –8)
3y² -22y –16
Factors of-48 Sum of –22 1, 2,
3y² -24y +2y –16
3y(y –8) +2(y –8)
(y -8)(3y +2)
Now practice !!!