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

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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