math

posted by .

wow. i can't believe i forget this from last year.

5x^2 - 11x - 36 = 0

theres some kind of box trick or somethin to factor that but i forget!

  • math -

    Three possible tricks

    1. quadratic equation

    2. complete the square

    3. factor it

  • math -

    factoring
    (5x+9)(x-4) = 0
    x = -9/5
    or
    x = 4

  • math -

    i'm french so i don't know how do u solve this equation in usa

    5x^2 - 11x - 36 = 0

    11²-4*5*(-36)=121+720=841
    sqrt(841)=29

    solutions are (11-29)/(2*5) and (11+29)/(2*5)

  • math -

    umm... i don't remember how to do any of it. like to factor and get the ( ) ( ) can you tell me how to do that

  • math -

    ooh.. damon how did you get the (5x+9)(x-4) = 0 that's what i need to know

  • math -

    complete the square
    5 x^2 - 11 x = 36
    x^2 - 11/5 x = 36/5
    x^2 -11/5 x + 121/100 = 36/5 + 121/100
    (x-11/10)^2 = 841/100
    x-11/10 = +/- 29/10

    x = (11+29)/10 = 4
    or
    x = (11-29)/10 = -18/10 = -9/5

  • math -

    Oh, ok, I guessed'
    Like so
    5 is only 5 * 1
    so there has to be something like
    (5x + ;;;) (x - :::)
    now the Plus and minus may change and 36 has factors
    2*2*3*3
    which could be 12 and 3
    or 6 and six
    or 9 and 4
    turns out 9 and 4 work

  • math -

    Tin tin is using some form of quadratic equation

    x = -b/2 +/- (1/2) sqrt{ b^2 - 4 a c}
    if in form
    a x^2 + b X + c = 0

  • Forgot a in denominator -

    x = -b/2a +/- (1/2a) sqrt{ b^2 - 4 a c}
    if in form
    a x^2 + b X + c = 0

  • math -

    By the way, the quadratic equation really is completing the square

    a x^2 + b x + c = 0

    x^2 + (b/a) x = -(c/a)

    x^2 + (b/a) x + (b/2a)^2 = -(c/a) +(b/2a)^2

    (x + b/2a)^2 = [-c/a + (b/2a)^2 ]
    so
    x+ b/2a = +/- sqrt (b^2/4a^2 - 4 a c/4a^2)
    or
    x = -b/2a +/- (1/2a)sqrt(b^2-4 a c)

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