What is Factoring?Date: 01/17/98 at 14:01:47 From: Melissa Subject: Factoring I do not get how to do this algebra. One of the sample questions is: x squared + 5x + 4 I don't understand how to start out factoring. Date: 01/20/98 at 21:15:47 From: Doctor Loni Subject: Re: Factoring Factoring can be a very intimidating thing, but once you get the hang of it you will be a pro. First, let me make sure you understand what factoring is. To factor anything, you get it into its smaller component parts, finding out what smaller parts make up the bigger part. For instance, when you factor a number, say 12, you break it into its smallest parts. You could start with 6 x 2. Then 6 can be factored into 2 x 3, so your factors of 12 could be 2 x 2 x 3. So to factor x^2 + 5x + 4 is to break it down into simpler parts. You may have already done some problems like this: (x + 3)(x - 4) Multiplying it out gives you: x^2 - 4x + 3x - 12 Simplifying: x^2 - x - 12 There are some very important things to notice here: you get the first term of the trinomial, x ^2 (they call it a trinomial because three terms are being added) by multiplying the FIRST terms in each parenthesis (x * x). The last number in the trinomial, the constant (meaning it has no x's in it) (-12) comes from multiplying the LAST terms in each parenthesis (3 * -4). To get the middle term (the x term, which in this case is - x), first multiply the first term in the first parenthesis (x) by the last term in the second parenthesis (-4) (we can call these terms the OUTSIDE terms because they are the outside terms of the two parentheses). Then multiply the last term in the first parenthesis (3) by the first term in the second parenthesis (x) (we can call the terms in the second parenthesis the INSIDE terms because they are the inside terms of the two parentheses). Then add the "inside" and the "outside" together. Thus the middle term becomes 3x - 4x = -x. You can also remember this by remembering the word F O I L. Foil stands for First, Outside, Inside, Last. To review: in (x + 3)(x - 4) the First is x* x or x^2, the Outside is x * (-4) (or -4x) the Inside is3 * x (or 3x) and the Last is (3) * (-4) (or -12) (x + 3) and (x - 4) are factors of x^2 - x - 12 Factoring a trinomial is just doing the reverse of what I just did in the problem above. You want to get from the trinomial back to the factors. First make sure your problem is written in the right way, that is, the term with x^2 comes first (the first term), the term with x (also called the middle term) comes next, and the constant - the term with no x in it - comes last. In your problem x^2 is the first term, 5x is the middle term, and 4 is the last term. Step 1 is to write the problem on your paper: x^2 + 5x + 4 Step 2 is to write two empty parentheses like this: ( )( ) We do this because we know the factors of a trinomial look this way. Step 3 is to look at the first term, which is x^2. We know from the problem I did above that the x^2 term comes from multiplying the first term in the first parenthesis by the first term in the second parenthesis. What are the only two things that can be multiplied together to give you x^2? That's right! x * x will give you x^2. You write it like this: (x )(x ) Step 4 is to look at signs. Remember from the problem above that the constant term (the one with no x in it) is found by multiplying the last terms of each parenthesis. Because the sign of the constant term in your problem (4) is plus, the signs of both the numbers that multiply together to give you 4 have to be either positive or negative (because a + * + or a - * - gives you a positive number, and a + * - will give you a negative number). So your parentheses could now look like this: (x - )(x - ) or this: (x + )(x + ) Look back at my previous problem to see what gave you a middle term. Remember you are adding the "outside" to the "inside." If both signs are minus, this will give you a minus number; if both signs are plus it will give you a plus number. Because +5x is positive, you know that the signs have to be positive. Thus you get: (x + )(x + ) Step 5 is the tricky part. You know that the last term is found by multiplying the last two numbers in each parenthesis together. This means the numbers could either be 4 and 1, or 2 and 2 (because 4 x 1 = 4 or 2 x 2 = 4). But you also know that the inside terms multiplied together plus the outside terms multiplied together will give you the middle term, so you have to try them out. Let's try 2 and 2: (x+ 2)(x +2) = x^2 +2x +2x + 4 = x^2 + 4x + 4 Whoops! The middle term is not right. We need 5x not 4x. Now let's try 4 and 1: (x + 4)(x + 1) = x^2 + 4x + 1x + 4 = x^2 + 5x +4 We got it. The factors of x^2 + 5x + 4 are (x + 4) and (x +1). Let's try another one: Factor x^2 + x - 2 Here are our parentheses: ( ) ( ) Remember F O I L (first, outside, inside, last) x^2 is the first terms multiplied together and the only thing they can be are x and x: (x )(x ) The sign in front of the constant is a minus (-2) so the signs have to be different (this is the only way we can get a -2, found by multiplying the last terms). (x + ) (x - ) Now, because the term in front of the x term (+x) is a plus we know that when we multiply the outside and inside terms together and add them to each other, we need to end up with a plus. Because the signs are different, the bigger term will have to be a plus. Now the only factors of 2 are 2 and 1 so they will be the only numbers we will have to worry about. If we put 2 and 1 in like this: (x +2) (x -1) = x^2 - x + 2x - 2 = x^2+ x - 2 If we had reversed the 2 and the 1 we would have ended up with -x instead of +x. That was a long explanation. The main things to remember are FOIL (first, outside, inside, last) and that sometimes it may take a little trial and error to get the right facts. Always multiply the factors back out to make sure you end up with what you started with. If you have more questions or need more help, let me know. -Doctor Loni, The Math Forum Check out our web site! http://mathforum.org/dr.math/ |
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