Welcome to another video from Ultimate Algebra.com. We are still on solving quadratic equations. This time we want to use the quadratic formula.

 

Here our work is just memorizing the formula and substituting the values of a, b and c.

 

The quadratic formula is x, equals, minus b, plus or minus, square root of b squared minus 4ac, all over 2a

 

Let's use this to solve the previous question,

X squared, plus 3x, minus 18, equals zero.

so here the coefficient of the x squared is 1, which is our a.

b is the coefficient of x, which is 3, and c is the constant, which is negative 18.

 

We put these values into the formula.

 

we have x, equals, minus 3, plus or minus, square root of 3 squared, minus 4 times 1, times negative 18, all over 2 times 1.

 

The 3 squared becomes 9.

The minus 18 times minus 4 becomes plus 72.

So we add these to get 81 and then find the square root.

Finding the square root gives 9.

We now have x equals, minus 3, plus or minus 9, all over 2.

 

We can solve the plus part as

x equals, minus 3, plus 9, all over 2.

This is 6 divided by 2.

Therefore, x equals 3.

 

Next, we solve the minus part.

X equals, minus 3, minus 9, all over 2.

This is negative 12 divided by 2.

Therefore, x equals, negative 6.

 

We see we got the same answers as the previous method.

 

There is one more way we can solve quadratic equation, which is by completing of squares. We will look at that later in this chapter.

 

Please check the examples and solve each question with both methods.

Have a great day. Good bye