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Math Help - this quadratic equation doesn't seem to factorise

  1. #1
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    this quadratic equation doesn't seem to factorise

    Hi guys,

    I've been trying to factorise the following equation:

    4p^2 + 12p + 9 = 0

    I can't seem to find any values for a and b so that 4a+b=12 and a*b=9

    Any help on this?

    thanks
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  2. #2
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    Quote Originally Posted by portia View Post
    Hi guys,

    I've been trying to factorise the following equation:

    4p^2 + 12p + 9 = 0

    I can't seem to find any values for a and b so that 4a+b=12 and a*b=9

    Any help on this?

    thanks
    (2p+3)(2p+3)
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  3. #3
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    Thanks, I do appreciate your answer, however, I'd like to know how you did it
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  4. #4
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    Quote Originally Posted by portia View Post
    Thanks, I do appreciate your answer, however, I'd like to know how you did it

    I find that explaining methods of factoring is very tricky... basically I take advantage of the fact that I had it drilled into me and I am quite quick with multiplying in my head

    But here's my shot at it

    4x^2+12x+9

    4 has the following factors 1,2,2,4

    So if this thing is factorable it has one of the two forms below

    1) (x )(4x ) or
    2) (2x )(2x )

    So I just pick one and run with it, let's say we guess wrong

    Try (x )(4x )

    Now 9 has the factors 1,3,3,9 so we can put 1,9 together in any order or 3,3 together

    Since 9 is positive, either both numbers are negative or both numbers are positive

    So our possible combinations are: (x+3)(4x+3), (x-3)(4x-3), (x+1)(4x+9), (x+9)(4x+1), (x-1)(4x-9), (x-9)(4x-1)

    I run through all these in my head and realize that I was not getting 12x out of the deal, so I move to the next one

    Try (2x )(2x )

    Now for blank space we have exactly the same combinations as above, I run through them in my head (and by this I mean I say to myself (2x+1)(2x+9) will give me 20x, nope no good try again)

    and I get that (2x+3)(2x+3) is my answer


    hope this helps

    and by the way, this method is significantly quicker than setting up those equations and looking at it once you get good with this
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  5. #5
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    Ok, I kind of understand it. I also used the quadratic formula and got -3/2 which makes sense:

    (x+3/2)(x+3/2) /// multiplied by 2

    (2x+3)(2x+3)

    So in this case there's only one solution: x=-3/2

    I'll need to practise it a lot more. I am ok when the coefficient 'a' is 1 as in x^2+4x........ but when 'a' is different to 1 then I sometimes struggle.

    thanks
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  6. #6
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    you can of course always apply the quadratic formula

    When you apply it, you get 2 answers in general, call them r_1 and r_2

    Then the quadratic can be written as (x-r_1)(x-r_2)

    In your case r_1=r_2=-\frac{3}{2}

    So if you were stuck on a test and had time this is a fail safe, but obviously factoring is preferred
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  7. #7
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    The quick way to see whether a quadratic will factorise (at least until you're good doing in mentally) is to find the discriminant, \Delta

    \Delta = b^2-4ac

    If \Delta is a perfect square it will factorise
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  8. #8
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    yeah but you can end up with fractions as the factors in which case I would just use the quadratic formula anyway
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  9. #9
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    Quote Originally Posted by portia View Post
    Hi guys,

    I've been trying to factorise the following equation:

    4p^2 + 12p + 9 = 0

    I can't seem to find any values for a and b so that 4a+b=12 and a*b=9

    Any help on this?

    thanks
    this is a complex trinomal, this means you have to multiply the A value with the C value and find two numbers that add up to 12 and multiply to 36. In this case it will be 6p and 6p. From there to find the great common factor between the first two values and the next two values. If you did it right you will get one value twice in your work.

    This is how your work should work:
    0 = 4p^2+12p+9
    0 = 4p^2+6p+6p+9
    0 = 2p(2p+3)+3(2p+3)
    0 = (2p+3)(2p+3)
    0 = (2p+3)^2

    Therefore (2p+3)^2 is 4p^2+12p+9=0.

    P cannot equal -3 over 2. The root is -3 over 2.
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