Posted by Anna-Marie on Monday, January 28, 2008 at 7:24pm.
Three possible tricks
1. quadratic equation
2. complete the square
3. factor it
factoring
(5x+9)(x-4) = 0
x = -9/5
or
x = 4
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)
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
ooh.. damon how did you get the (5x+9)(x-4) = 0 that's what i need to know
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
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
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
x = -b/2a +/- (1/2a) sqrt{ b^2 - 4 a c}
if in form
a x^2 + b X + c = 0
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)