How do you solve x^2 - 3x - 4 = 0 using the quadratic formula?

1 Answer
Tiffany Quach
May 19, 2016

x=-1
x=4

Explanation:

The quadratic formula is (-b+-sqrt(b^2-4ac))/(2a).

In a trinomial written in the form ax^2+bx+c,
a- the coefficient (number) in front of the x^2
b- the coefficient in front of the x
c- the constant (number by itself and has no variable- in this case, has no x- attached to it

So, in this problem,
a= 1 (no coefficient is an invisible 1)
b= -3 (don't forget about the negative!)
c= -4

Now, we plug all these values into the quadratic formula with the corresponding values:
(--3+-sqrt((-3)^2-4(1)(-4)))/(2(1))
(3+-sqrt(9-4(-4)))/(2)
(3+-sqrt(9+16))/(2)
(3+-sqrt(25))/(2)
(3+-5)/(2)

Now, you have to solve for BOTH solutions:

One solution: x= (3+5)/2 = 8/2 = 4

Second solution: x= (3-5)/2 = -2/2 = -1

So, the two solutions are x= -1 and x=4

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