Homework Help: Factoring Trinomials

Posted by -Untamed- on Monday, October 31, 2011 at 6:40pm.

Working on how to factor trinomials, and the method that works best for me is the criss cross method, so Ive sticked with it. But Ive been stuck on one question for the past few minutes, and don't know what I've been doing wrong.

4x^2+23x+15

2x and 2x times to make a 4x. 5 and 3 times to make 15. And Im left with 10x and 6x, and they are supposed to add to make a 23x. But they are not:\ Ive even tried flipping them, but there's no point because the numbers are the same.

Factoring Trinomials - Damon, Monday, October 31, 2011 at 6:48pm
try
(x-5)(4x-3)
Factoring Trinomials - Damon, Monday, October 31, 2011 at 6:51pm
Now to do that I solved the quadratic equation (google that)
I found for solutions
x = 5 and x = 6/8 which is 3/4
so
(x-5)(x-3/4) = 0 is solution
I can multiply both sides by 4 to get rid of the faction
(x-5)(4x-3) = 0
Factoring Trinomials - -Untamed-, Monday, October 31, 2011 at 6:52pm
How did you get that using the criss cross method?
Factoring Trinomials - Damon, Monday, October 31, 2011 at 6:54pm
solving for zeros with quadratic equation:
[ -23 +/- sqrt (529 - 240) ]/8

[-23+/-17]/8

-40/8 and -6/8
Factoring Trinomials - Damon, Monday, October 31, 2011 at 6:55pm
I never heard of the criss cross method. I used the quadratic equation to solve for zeros of the quadratic expression.
Factoring Trinomials - Damon, Monday, October 31, 2011 at 6:58pm
5 and 4 make 20
1 and 3 make 3
Factoring Trinomials - Damon, Monday, October 31, 2011 at 7:00pm
Do it backwards from my solution and you will see :)
Factoring Trinomials - -Untamed-, Monday, October 31, 2011 at 7:02pm
For the criss cross method, Im supposed to find two numbers that multiply to get 4x^2. And those were 2x and 2x. Then, Im supposed to find two numbers that multiply to give me 15, which was either 15 and 1 or 3 and 5. I used both, by criss crossing 2x to 3 and 2x to 5. That gave me 10x and 6x which does not give me 23x, and same with using 15 and 1, that just gave me 28, -28, etc.
Factoring Trinomials - -Untamed-, Monday, October 31, 2011 at 7:07pm
Nevermind, I think I got it. Thanks for showing me the Quadratic way.
Factoring Trinomials - Damon, Monday, October 31, 2011 at 7:15pm
But 1 * 4 = 4 as well :)
Factoring Trinomials - t, Monday, December 19, 2011 at 5:32pm
b-6b+8

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