document.write( "Question 299480: reverse foil\r
\n" ); document.write( "\n" ); document.write( "x^2-2x-15 \n" ); document.write( "

Algebra.Com's Answer #215169 by jim_thompson5910(35256) $\"\"$ $\"About$

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\n" ); document.write( "\n" ); document.write( "Looking at the expression $\"x%5E2-2x-15\"$ , we can see that the first coefficient is $\"1\"$ , the second coefficient is $\"-2\"$ , and the last term is $\"-15\"$ .\r
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\n" ); document.write( "\n" ); document.write( "Now multiply the first coefficient $\"1\"$ by the last term $\"-15\"$ to get $\"%281%29%28-15%29=-15\"$ .\r
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\n" ); document.write( "\n" ); document.write( "Now the question is: what two whole numbers multiply to $\"-15\"$ (the previous product) and add to the second coefficient $\"-2\"$ ?\r
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\n" ); document.write( "\n" ); document.write( "To find these two numbers, we need to list all of the factors of $\"-15\"$ (the previous product).\r
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\n" ); document.write( "\n" ); document.write( "Factors of $\"-15\"$ :\r
\n" ); document.write( "\n" ); document.write( "1,3,5,15\r
\n" ); document.write( "\n" ); document.write( "-1,-3,-5,-15\r
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\n" ); document.write( "\n" ); document.write( "Note: list the negative of each factor. This will allow us to find all possible combinations.\r
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\n" ); document.write( "\n" ); document.write( "These factors pair up and multiply to $\"-15\"$ .\r
\n" ); document.write( "\n" ); document.write( "1*(-15) = -15
\n" ); document.write( "3*(-5) = -15
\n" ); document.write( "(-1)*(15) = -15
\n" ); document.write( "(-3)*(5) = -15\r
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\n" ); document.write( "\n" ); document.write( "Now let's add up each pair of factors to see if one pair adds to the middle coefficient $\"-2\"$ :\r
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First Number	Second Number	Sum
1	-15	1+(-15)=-14
3	-5	3+(-5)=-2
-1	15	-1+15=14
-3	5	-3+5=2

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\n" ); document.write( "\n" ); document.write( "From the table, we can see that the two numbers $\"3\"$ and $\"-5\"$ add to $\"-2\"$ (the middle coefficient).\r
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\n" ); document.write( "\n" ); document.write( "So the two numbers $\"3\"$ and $\"-5\"$ both multiply to $\"-15\"$ and add to $\"-2\"$ \r
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\n" ); document.write( "\n" ); document.write( "Now replace the middle term $\"-2x\"$ with $\"3x-5x\"$ . Remember, $\"3\"$ and $\"-5\"$ add to $\"-2\"$ . So this shows us that $\"3x-5x=-2x\"$ .\r
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\n" ); document.write( "\n" ); document.write( " $\"x%5E2%2Bhighlight%283x-5x%29-15\"$ Replace the second term $\"-2x\"$ with $\"3x-5x\"$ .\r
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\n" ); document.write( "\n" ); document.write( " $\"%28x%5E2%2B3x%29%2B%28-5x-15%29\"$ Group the terms into two pairs.\r
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\n" ); document.write( "\n" ); document.write( " $\"x%28x%2B3%29%2B%28-5x-15%29\"$ Factor out the GCF $\"x\"$ from the first group.\r
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\n" ); document.write( "\n" ); document.write( " $\"x%28x%2B3%29-5%28x%2B3%29\"$ Factor out $\"5\"$ from the second group. The goal of this step is to make the terms in the second parenthesis equal to the terms in the first parenthesis.\r
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\n" ); document.write( "\n" ); document.write( " $\"%28x-5%29%28x%2B3%29\"$ Combine like terms. Or factor out the common term $\"x%2B3\"$ \r
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\n" ); document.write( "\n" ); document.write( "Answer:\r
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\n" ); document.write( "\n" ); document.write( "So $\"x%5E2-2x-15\"$ factors to $\"%28x-5%29%28x%2B3%29\"$ .\r
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\n" ); document.write( "\n" ); document.write( "In other words, $\"x%5E2-2x-15=%28x-5%29%28x%2B3%29\"$ .\r
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\n" ); document.write( "\n" ); document.write( "Note: you can check the answer by expanding $\"%28x-5%29%28x%2B3%29\"$ to get $\"x%5E2-2x-15\"$ or by graphing the original expression and the answer (the two graphs should be identical).
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