How do you solve 5m^2 - 11m - 3 = 0 using the quadratic formula?

1 Answer
Oct 16, 2015

First, we determine the values of a, b, and c that go into the quadratic formula, then plug them in and solve...

Explanation:

So, when we have a quadratic equation:

ax^2 + bx + c = 0

One of the ways we can solve for x is by using the quadratic formula:

![https://www.blendspace.com/lessons/fk5Tc7PpFlsQWg/the-quadratic-formula](useruploads.socratic.org)

But wait... for your problem, there's no x! Taking a look at the equation, the first thing we need to do is realize that x is just a placeholder for any variable (a number that we don't know... yet). In your question, the variable is shown as m. It's the same thing; we could call it x or m or omega or whatever we like.

Now, we need to figure out the values of a, b, and c. a is the number before the x^2 variable (or, in your case, m^2). b is the number before m, and c is the number not associated with the variable. So...

a = 5

b = -11 (don't forget that negative sign!)

c = -3 (again, the negative sign is important)

Now, we can take those three numbers and plug them into the quadratic equation:

x = (-(-11) (+-) sqrt((-11^2)- (4)(5)(-3))/((2)(5)))

See that +- in there? That means we're going to get two answers.

Either

x = (-(-11) (+) sqrt((-11^2)- (4)(5)(-3))/((2)(5)))

OR

x = (-(-11) (-) sqrt((-11^2)- (4)(5)(-3))/((2)(5)))

x can either be 2.445 or -0.2453 (leaving off quite a few decimal places)

Let's check to make sure that's right, by plugging in our values for x:

Does (5)(2.445^2) + (-11)(2.445) + (-3) = 0?

Does (5)(0.2453^2) + (-11)(0.2453) + (-3) = 0?