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Thread: maths homework (factorising quadratics a not=1)

  1. #1 maths homework (factorising quadratics a not=1) 
    Forum Sophomore somfooleishfool's Avatar
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    5Y^2-14Y-3

    I don't care for the answer to this, I already know the answer is =(Y-3)(5Y+1)

    I need to know how to figure this out by Tuesday or I'm screwed for uni


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  3. #2  
    Forum Sophomore somfooleishfool's Avatar
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    Ill demonstrate where Im stuck (I'v been working it out all myself without help, following my maths book's instructions)

    "consider 5Y^2 - 14Y - 3

    to factorise

    1. multiply the first and last terms together (understood) {- 15Y^2}

    2. find two factors of -15Y^2 that add to give the middle term - 14Y (understood) {-15Y and Y}

    3. replace the middle term with these two values (understood)
    {5Y^2 - 15Y + y - 3}

    4. factorise the first two and the last two terms using a common factor
    (not understood)
    4a. {5Y^2 - 14Y - 3} (starting equation)
    4b. {5Y^2 - 15Y + Y -3} (3rd step)
    4c. {5Y(y - 3) + 1 (Y - 3)} (wtf how do u get this)


    5. take out (Y - 3) as a common factor (not considered yet as step 4 not understood) {(Y - 3)(5y + 1)

    edit: had one of the "y"s written as an x.


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  4. #3  
    Forum Radioactive Isotope MagiMaster's Avatar
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    5Y^2 and -15Y have a common factor of 5Y. Divide that out and you get 5Y * (Y - 3)
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  5. #4  
    Forum Sophomore somfooleishfool's Avatar
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    Quote Originally Posted by MagiMaster
    5Y^2 and -15Y have a common factor of 5Y. Divide that out and you get 5Y * (Y - 3)
    Im nearly there. I dont see where the +1 comes from though

    Thanks a whole bunch. Your help is greatly appreciated
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  6. #5  
    Forum Sophomore somfooleishfool's Avatar
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    I GOT IT!!!! yusss lol im not stuffed on tuesday . the +1 obviously is the comon factor of the last two numbers as it clearly says -_-.

    I hate maths. When your trying to figure it out its like the lecturer is talking at a brick wall.... nothing soaks in. But then when you get it its like OHHHHHHH thats so nubby nd easy why am I so stupid?
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  7. #6  
    Forum Sophomore somfooleishfool's Avatar
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    ok so I thought I had it but then I tried to do another example and I fail.

    4y^2+8y-5

    so far what iv attempted

    4y^2 X -5 =20y^2

    20 has factors 1-2, 2-10, 4-5

    2-10 work out because -2+10 = 8

    so now we rewrite original problem
    4y^2 - 2y + 10y - 5

    now im a bit stuck but this has been so far my best attempt

    -2Y(-2Y -1) -5(-2Y -1) close but no cigar. it all works out except the
    {-5X -1=5 not -5} Im wondering if I havnt forgotten something really stupid.
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  8. #7  
    Forum Ph.D. Heinsbergrelatz's Avatar
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    ------------------




    "Mathematicians stand on each other's shoulders."- Carl Friedrich Gauss


    -------------------
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  9. #8  
    Forum Sophomore somfooleishfool's Avatar
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    Quote Originally Posted by Heinsbergrelatz
    you skipped a step. Im still trying to figure out how to do the bit before that.
    refur to "Give a man a fish and you feed him for a day. Teach a man to fish and you feed him for a lifetime."
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  10. #9  
    Forum Sophomore somfooleishfool's Avatar
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    can somone work out like 3 examples for me? with steps included prefurably. part of my problem is I only have 1 worked out example to go by and its easy to make assumptions based on 1 example that are incorrect.
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  11. #10  
    Forum Sophomore somfooleishfool's Avatar
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    Ive finally figured it out completely and I can complete examples from scratch. DONE
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  12. #11  
    Forum Sophomore somfooleishfool's Avatar
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    I just realized I forgot all about step 5.

    "5. take out (Y - 3) as a common factor (not considered yet as step 4 not understood) {(Y - 3)(5y + 1)"

    it seems to me you just take away one of the brakets and put the two numbers outside of the brackets into brackets. Is this correct? would someone care to explain how this works?

    as far as I can see this would get expanded to as follows
    y * 5y= 5y^2
    y * 1 = y
    -3 * 5y- =-15y
    -3 * 1 = -3

    so 5y^2+y-15y -3
    5y^2+14y -3 (WOOT correcto)

    so I wasnt sure whether to post this or not seeing as I figured it out in the process of typing it out lol but why not
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