Geometry

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Solve the quadratic equation.
13) x2 - x = 72
A) -8, 9
B) -8, -9
C) 1, 72
D) 8, 9

Can some one please check my answer and let me know if I did it the right way.
x2-x-72=0
(x-9)(x-8)=0
so the answer is D
8,9

  • Geometry -

    no, your factors should have been
    (x-9)(x+8) = 0

    so x=9 or x=-8 which would be A

  • Geometry -

    Well you see that was how I did at first but I was not too sure about it so then I got it right the first time I always get confused with that any tips to remind me next time. Thanks

  • Geometry -

    After you factor, you should multiply it back out to see if you end up with the original.
    I knew immediately that your factors were wrong.
    Both your factors ended with negatives for the constants, so your last term would have been positive, but it was -72

  • Geometry -

    Oh,I see thank you I have another one for you that I don't know how to start it and it goes like this 6y2 = 7y - 5?
    Can you please help me just to start it off

  • Geometry -

    put everything to one side
    6y^2 - 7y + 5 = 0

    I see a positive at the end, which tells me that the signs are the same, looking at the middle term they must be both negative.
    so it must be (?x - 1)(?x - 5) = 0

    only 4 possibilies are
    (2x-1)(3x-5) = 0
    (3x-1)(2x-5) = 0
    (6x-1)(x-5) or
    (x-1)(6x-5) = 0

    unfortunately none of these give us a middle term of -7y

    so your question does not factor.

  • Geometry -

    So then the answer is 0 because like you mention there is no factor "correct"

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