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## Pythagorean theorem

# Pythagorean theorem with isosceles triangle

CCSS.Math:

## Video transcript

- [Instructor] We're asked
to find the value of x in the isosceles triangle shown below. So that is the base of this triangle. So pause this video and see
if you can figure that out. Well the key realization to solve this is to realize that this
altitude that they dropped, this is going to form a right angle here and a right angle here and notice, both of these triangles, because this whole thing
is an isosceles triangle, we're going to have two
angles that are the same. This angle, is the same as that angle. Because it's an isosceles triangle, this 90 degrees is the
same as that 90 degrees. And so the third angle
needs to be the same. So that is going to be the same as that right over there. And since you have two
angles that are the same and you have a side between them that is the same this altitude of 12 is on both triangles, we know that both of these
triangles are congruent. So they're both going to have 13 they're going to have one side that's 13, one side that is 12 and so this and this side are going to be the same. So this is going to be x over two and this is going to be x over two. And so now we can use that information and the fact and the Pythagorean Theorem to solve for x. Let's use the Pythagorean Theorem on this right triangle on the right hand side. We can say that x over two squared that's the base right over here this side right over here. We can write that x over two squared plus the other side plus 12 squared is going to be equal to
our hypotenuse squared. Is going to be equal to 13 squared. This is just the Pythagorean Theorem now. And so we can simplify. This is going to be x. We'll give that the same color. This is going to be x squared over four. That's just x squared over two squared plus 144 144 is equal to 13 squared is 169. Now I can subtract 144 from both sides. I'm gonna try to solve for x. That's the whole goal here. So subtracting 144 from both sides and what do we get? On the left hand side, we have x squared over four is equal to 169 minus 144. Let's see, 69 minus 44 is 25. So this is going to be equal to 25. We can multiply both sides by four to isolate the x squared. And so we get x squared
is equal to 25 times four is equal to 100. Now, if you're just looking
this purely mathematically and say, x could be
positive or negative 10. But since we're dealing with distances, we know that we want the
positive value of it. So x is equal to the principle root of 100 which is equal to positive 10. So there you have it. We have solved for x. This distance right here, the whole thing, the whole thing is
going to be equal to 10. Half of that is going to be five. So if we just looked at
this length over here. I'm doing that in the
same column, let me see. So this length right over here, that's going to be five and indeed, five squared plus 12 squared, that's 25 plus 144 is 169, 13 squared. So the key of realization here is isosceles triangle, the altitudes splits it into
two congruent right triangles and so it also splits this base into two. So this is x over two and this is x over two. And we use that information and the Pythagorean Theorem to solve for x.