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Math Help - And now what?

  1. #1
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    And now what?

    I have to factor the following polynomial completely. But I don't have the answer so I don't know if I did it or it can be factored further.

    <br />
16x^2+16x-1<br />

    <br />
8x(2x+2)-1<br />

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  2. #2
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    Finding this polynomial roots will allow you to factor it completely.
    So the problem is to find the roots.






    Try before looking at the solution (which can be wrong):

    16x^{2}+16x-1=16(x+\frac{1+\sqrt{\frac{5}{4}}}{2})(x+\frac{1-\sqrt{\frac{5}{4}}}{2})
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  3. #3
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    Wink

    I think the answer has to be a lot easier, the book i'm using hasn't aproached roots so far. I'm just factoring polynomials. Said that, I don't understand the point of findig the roots. I looked at your explanation and...well, maybe the book author meant "factor completly" according to what we have covered so far...Anyway, I'll keep trying.

    I notice the second part looks very much like a difference of squares.

    <br />
a^2-b^2=(a+b)(a-b)<br />



    ...And how do I find roots if that were the only chance? Prease an explanation
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  4. #4
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    Quote Originally Posted by Alienis Back View Post
    I have to factor the following polynomial completely. But I don't have the answer so I don't know if I did it or it can be factored further.

    <br />
16x^2+16x-1<br />

    <br />
8x(2x+2)-1<br />
Mr F says: This will not help.

    Note that 16x^2 + 16x - 1 = (4x+2)^2 - 5 = (4x + 2)^2 - (\sqrt{5})^2.

    Now use the difference of two squares formula.
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  5. #5
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    Ok! This works for me but how did you know that 16x^2 + 16x - 1 would be equivalent to (4x+2)^2 - 5
    I mean...as it is not a perfect square trinomial I couldn't try finding two factors adding to the product of the external values as I did with this:

    <br />
16x^5 - 44x^4 +30x^3<br />

    <br />
2x^3 (8x^2 - 22x + 15)<br />

    I usually find the numerical product of the external values:

    <br />
8(15)=120<br />
    And then try to find two factors of 120 that add to the middle term. i.e. <br />
-22<br />

    <br />
-10(-12)=120<br />

    Finally all I have to do is just replacing:

    <br />
2x^3 (8x^2-10x-12x+15)<br />

    And solve...

    <br />
2x^3 [2x(4x-5)-3(4x-5)]<br />

    <br />
2x^3 (4x-5) (2x-3)<br />

    And that's it!

    As you see this aproach wouldn't have worked with 16x^2 + 16x - 1 as it isn't a perfect square. Now my question is how can I know (4x+2)^2 - 5 will solve for the above trinomial? Is there any special formula or technique?
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  6. #6
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    Quote Originally Posted by Alienis Back View Post
    Ok! This works for me but how did you know that 16x^2 + 16x - 1 would be equivalent to (4x+2)^2 - 5
    I mean...as it is not a perfect square trinomial I couldn't try finding two factors adding to the product of the external values as I did with this:

    <br />
16x^5 - 44x^4 +30x^3<br />

    <br />
2x^3 (8x^2 - 22x + 15)<br />

    I usually find the numerical product of the external values:

    <br />
8(15)=120<br />
    And then try to find two factors of 120 that add to the middle term. i.e. <br />
-22<br />

    <br />
-10(-12)=120<br />

    Finally all I have to do is just replacing:

    <br />
2x^3 (8x^2-10x-12x+15)<br />

    And solve...

    <br />
2x^3 [2x(4x-5)-3(4x-5)]<br />

    <br />
2x^3 (4x-5) (2x-3)<br />

    And that's it!

    As you see this aproach wouldn't have worked with 16x^2 + 16x - 1 as it isn't a perfect square. Now my question is how can I know (4x+2)^2 - 5 will solve for the above trinomial? Is there any special formula or technique?
    I got the expression by completing the square:

    16x^2 + 16x - 1 = 16\left(x^2 + x - \frac{1}{16}\right)

     = 16 \left( \left[x + \frac{1}{2}\right]^2 - \frac{1}{4} - \frac{1}{16}\right)

    = 16\left(x + \frac{1}{2}\right)^2 - 4 - 1

    = 4^2 \left( x + \frac{1}{2} \right)^2 - 5

    = \left( 4x + 4 \cdot \frac{1}{2} \right)^2 - 5

    = \left( 4x + 2 \right)^2 - 5.

    Now compare this with the difference of two squares formula A^2 - B^2 = (A - B)(A + B):

    Substitute A = 4x + 2 and B = \sqrt{5}.
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  7. #7
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    Then here is how it works. You factored out the 16:

    16x^2 + 16x - 1 = 16\left(x^2 + x - \frac{1}{16}\right)

    = 16 \left[ \left(x + \frac{1}{2}\right)^2 - \frac{1}{4} - \frac{1}{16}\right]

    __________________________________________________ ________________

    But I didn't know where you obtained that \left(x+\frac{1}{2}\right)^2 so I solved...

    (x+\frac{1}{2}) (x+\frac{1}{2})

    =x^2+\frac{x}{2}+\frac{x}{2}+\frac{1}{4}

    =x^2+x+\frac{1}{4}

    And replaced...

    =16 [x^2+x+\frac{1}{4}-\frac{1}{4}-\frac{1}{16}]

    =16\left[x^2+x-\frac{1}{16}\right]

    __________________________________________________ ________________

    So now I continue...

    =16\left[\left(x+\frac{1}{2}\right)^2-\frac{1}{4}-\frac{1}{16}\right]

    =16\left[\left(x+\frac{1}{2}\right)^2-\frac{4}{16}-\frac{1}{16}\right]

    and the 16 outside the parentesis will multiply for \frac{4}{16} and \frac{1}{16} so we now have...

    =16\left(x+\frac{1}{2}\right)^2-4-1

    =4^2\left(x+\frac{1}{2}\right)^2-4-1

    =4^2\left(x+\frac{1}{2}\right)^2-5

    Consider... (a^n) (b^n) = (ab)^n

    =(4x+\frac{4}{2}\right)^2 - 5

    =(4x+2)^2-5

    =(4x+2)^2-(\sqrt{5})^2

    And finally we apply the formula for the difference of two squares...

    =[(4x+2)-\sqrt{5}] [(4x+2)+\sqrt{5}]

    And that's it.

    Now...there is just one little thing that I don't understand. Hope you don't think I'm a pain in the... but how did you get that -\frac{1}{4} up there on the second stage?. I mean...It seems you just put it there arbitrarily so the whole thing could be worked out. Finally I want to thank you. Sincerely.
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  8. #8
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    Quote Originally Posted by Alienis Back View Post
    [snip]
    Now...there is just one little thing that I don't understand. Hope you don't think I'm a pain in the... but how did you get that -\frac{1}{4} up there on the second stage?. I mean...It seems you just put it there arbitrarily so the whole thing could be worked out. Finally I want to thank you. Sincerely.
    It's just my way of completing the square. You look like you've got your equally good way of completing the square which works well for you. Neither way is better of worse than the other. Whatever floats your boat .....
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