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Current time:0:00Total duration:4:43

- [Voiceover] We're asked to
simplify the expression by removing all factors
that are perfect squares from inside the radicals
and combining the terms. So, let's see if we can
do it and pause the video and give a-go at it
before we do it together. Alright, so let's see how we
can re-write these radicals. So, four times the square root of 20. Well, thats the same thing
as four times the square root of four, times the square root of five, because 20 is the same
thing as four times five. And 45, that's the same
thing as nine times five. And the reason why I'm
thinking about the four and I'm thinking about the nine is because those are perfect squares, so I could write this as, four times the square root of four times the square root of five. And then I could say, this
part right over here is minus three, times the
square root of nine, times the square root of five. The square root of 45
is the same thing as the square root of nine times five,
which is the same thing as square root of nine times
the square root of five, and then all of that is going to be over, all of that is over the square root of 35. Now, are there any perfect squares in 35? 35 is seven times five. No, neither of those are perfect squares. So, I could just leave
that as square root of 35. And, let's see, the square root of four? Well, thats going to be two. This is the principal root,
so we're thinking about the positive square root. The square root of nine is three. And so, this part right over is going to be four times two, times the square root of
five, so it's going to be eight square roots of five. And then, this part
over here is going to be minus three, times three,
times the square root of five. So, minus nine square roots of five. And all of that is going to
be over the square root of 35. Square root of 35. And, so let's see, if I
have eight of something, and I subtract nine of that something, I'm gonna have negative
one of that something. So, I could say negative one
times the square root of five, or I could just say negative
square root of five. Negative square root of five
over the square root of 35. I actually think I could
simplify this even more, because this is the same thing. This is equal to the
negative of the square root of five over 35. Both the numerator and the denominator are divisible by five. So, we could divide them both by five, and we would get the square root of divide the numerator by five, you get one. Divide the denominator
by five, you get seven. So, we can view this as
the square root of 1/7th. Square root of 1/7th. And we are all done. Let's do another one of these. These are strangely, strangely fun. And once again, pause it, and see if you can work
it out on your own. Perform the indicated operations. Alright, so let's first multiply. So, this essentially is doing the distributive property twice. And, actually let me just do it that way, so let's distribute the
square root of five, plus the square root of six. Let's first multiply it times
the square root of five. So, the square root of five
times the square root of five is going to be five. Square root of five times
the square root of six is the square root of 30. So, five plus the square root of 30. And then, when I take this expression, and I multiply it times the second term, times the negative square root of six. Well, negative square root of six times the square root of five is going to be the negative of the square root of 30. And then the negative of
the square root of six times the square root
of six is going to be, we're gonna subtract six. The square root of six times
the square root of six is six, then we have the negative out there. And so, just like that, we are
left with, well, let's see. Square root of 30 minus
the square root of 30, well those cancel out, thats zero. And we're left with five minus six, which is going to be
equal to negative one. And we are, we're, we're all done. Now, another way that
you could've viewed this, is you could've seen a pattern here. You could've said, Well, this is the same thing
as A plus B, times A minus B, or A is square root of five
and B is square root of six, and we know that this will result in the difference of squares. This is will be A squared minus B squared. And, so this for this
particular case it would be square root of five squared, minus square root of six squared. Which is of course
equal to five minus six, which is equal to negative one. Either way, hopefully you found
that vaguely entertaining.