 solve the equation, negative x squared plus 8x is equal to 1. Now, in order to really use the
quadratic equation, or to figure out what our a’s, b’s and
c’s are, we have to have our equation in the form, ax
squared plus bx plus c is equal to 0. And then, if we know our a’s,
b’s, and c’s, we will say that the solutions to this equation
are x is equal to negative b plus or minus the square root
of b squared minus 4ac– all of that over 2a. So the first thing we have to
do for this equation right here is to put it
in this form. And on one side of this
equation, we have a negative x squared plus 8x, so that looks
like the first two terms. But our constant is on
the other side. So let’s get the constant on the
left hand side and get a 0 here on the right hand side. So let’s subtract 1 from both
sides of this equation. The left hand side of the
equation will become negative x squared plus 8x minus 1. And then the right hand
side, 1 minus 1 is 0. Now we have it in that form. We have ax squared
a is negative 1. So let me write this down.
a is equal to negative 1. a is equal to negative 1. It’s implicit there, you could
put a 1 here if you like. A negative 1. Negative x squared is the same
thing as negative 1x squared. b is equal to 8. So b is equal to 8, that’s
the 8 right there. And c is equal to negative 1. That’s the negative
1 right there. So now we can just apply
the quadratic formula. The solutions to this equation
are x is equal to negative b. Plus or minus the square root
of b squared, of 8 squared, minus 4ac– let me do it in that
green color –minus 4, the green is the part
of the formula. The colored parts are the things
that we’re substituting into the formula. Minus 4 times a, which is
negative 1, times negative 1, times c, which is
also negative 1. And then all of that– let me
extend the square root sign a little bit further –all
of that is going to be over 2 times a. In this case a is negative 1. So let’s simplify this. So this becomes negative 8, this
is negative 8, plus or minus the square root
of 8 squared is 64. And then you have a negative 1
times a negative 1, these just cancel out just to be a 1. So it’s 64 minus is 4. That’s just that 4 over there. All of that over negative 2. So this is equal to negative
8 plus or minus the square root of 60. All of that over negative 2. And let’s see if we can
simplify the radical expression here, the
square root of 60. Let’s see, 60 is equal
to 2 times 30. 30 is equal to 2 times 15. And then 15 is 3 times 5. So we do have a perfect
square here. We do have a 2 times
2 in there. It is 2 times 2 times
15, or 4 times 15. So we could write, the square
root of 60 is equal to the square root of 4 times the
square root of 15, right? The square root of 4 times
the square root of 15, that’s what 60 is. 4 times 15. And so this is equal to– square
root of 4 is 2 times the square of 15. So we can rewrite this
expression, right here, as being equal to negative 8 plus
or minus 2 times the square root of 15, all of that
over negative 2. Now both of these terms right
here are divisible by either 2 or negative 2. So let’s divide it. So we have negative 8 divided
by negative 2, which is positive 4. So let me write it over here. Negative 8 divided by negative
2 is positive 4. And then you have this
weird thing. Plus or minus 2 divided
by negative 2. And really what we have
here is 2 expressions. But if we’re plus 2 and we
divide by negative 2, it will be negative 1. And if we take negative 2 and
divide by negative 2, we’re going to have positive 1. So instead of plus or minus, you
could imagine it is going to be minus or plus. But it’s really the same thing. Right? It’s really now minus or plus. If it was plus, it’s now
going to be a minus. If it was a minus, it’s now
going to be a plus. Minus or plus 2 times the
square root of 15. Or another way to view it is
that the two solutions here are 4 minus two roots of 15,
and 4 plus two roots of 15. These are both values of x
that’ll satisfy this equation. And if this confuses you, what I
did, turning a plus or minus into minus plus. Let me just take a little
bit of an aside there. I could write this expression
up here as two expressions. That’s what the plus
or minus really is. There’s a negative 8 plus 2
roots of 15 over negative 2. And then there’s a negative
8 minus 2 roots of 15 over negative 2. This one simplifies to–
negative 8 divided by negative 2 is 4. 2 divided by negative
2 is negative 1. 2 times a 4 minus the
square root of 15. And then over here you have
negative 8 divided by negative 2, which is 4. And then negative 2 divided by
negative 2, which is plus the square of 15. And I just realized I made
a mistake up here. When we’re dividing a 2 divided
by negative 2, we don’t have this 2 over here. This is just a plus or
minus the root of 15. We just saw that when
I did it out here. So this is minus the
square root of 15. And this is plus the
square root of 15. So the two solutions for this
equation– It’s good that I took that little hiatus there,
that little aside there. The two solutions could be 4
minus the square root of 15, or x, or and, x could be 4 plus
the square root of 15. Either of those values of x
will satisfy this original quadratic equation.

### 14 thoughts on “Example 2: Using the quadratic formula | Quadratic equations | Algebra I | Khan Academy”

• September 12, 2010 at 1:47 pm

thanks for explaining !

• October 14, 2010 at 1:02 am

You make this look like shooting fish in the barrel xD

• May 20, 2011 at 1:48 am

thanks

• June 18, 2011 at 1:29 am

I thought the formula for when "b" is an even number, it's "x= -b' squared plus or minus the square root of b' squared minus ac over a"?? so wouldn't the formula be "ax squared + 2b'x +c= 0 when a doesn't equal 0"

• June 18, 2011 at 1:30 am

wouldn't that be the quadratic formula II? 8 is an even number, so i thought you use the quad. formula 2 for when the coefficient of x is an even number, and use the quadratic formula 1 when the coefficient of x is an odd number.

• August 18, 2011 at 3:45 pm

so easy
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• August 18, 2011 at 3:45 pm

so easy
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• August 18, 2011 at 3:46 pm

so easy
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• December 2, 2011 at 8:17 am

Breaking down the square was very helpfull info. Thanks again!

• October 2, 2012 at 8:12 pm

could someone please explain why let's say

-8 + 2 all over -2 is 4-1 and not 4 + 2… wouldn't the -2 as the denominator cancel out with the -8 and become 4? why is it not so?

• October 10, 2016 at 6:50 pm

This man is a genius! 😀

• December 4, 2017 at 6:12 pm

it really helped me with my homework i subscribed.

• October 4, 2018 at 7:48 pm
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