If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains ***.kastatic.org** and ***.kasandbox.org** are unblocked.

Main content

Current time:0:00Total duration:4:35

CCSS.Math:

- [Instructor] We're told that Lainey runs a string of lights from
the ground straight up to a door frame that is 2.5 meters tall. Then they run the rest of the string in a straight line to
a point on the ground that is six meters from
the base of the door frame. There are 10 lights per
meter of the string. How many total lights are on the string? So I'll pause this video and see if you can work this out. All right, now let's work
through this together. I think this one warrants
some type of a diagram. So let me draw this door
frame that looks like this. And this door frame is 2.5 meters tall. So that's its height right over there. And what they're going to do, what Lainey's doing is she
is string up these lights, so that's this yellow right over here. So it goes up to the
top of that door frame and then they run the rest of the light in a straight line to
a point on the ground that is six meters from
the base of the door frame. So let me show a point that is six meters from the base of the door frame. So it looks something
like, maybe like that. So this distance right
over here is six meters. So they run the rest of the light from the top of the door frame to that point to that six meters away. So the yellow right over here, that is the light. And so we had to figure out how many total lights are on the string. So the way I would tackle
it is first of all, I wanna figure out how
long is the total string. And to figure that out, I just need to figure out, okay, it's gonna be 2.5 plus
whatever the hypothenuse is of this right triangle. I think it's safe to assume
that this is a standard house where door frames are at a
right angle to the floor. And so we have to figure out the length of this hypothenuse and if we know that, plus this 2.5 meters, then we know how long the
entire string of lights are. And then we just have to
really multiply it by 10 because there's 10 lights
per meter of the string. So let's do that. So how do we figure out
the hypothenuse here? Well, of course, we would
use Pythagorean theorem. So let's call this, and also call this h for hypothenuse. We know that the hypothenuse squared is equal to 2.5 squared plus six squared. So this is going to be
equal to 6.25 plus 36, which is equal to 42.25 or we could say that the hypothenuse is going to be equal to the square root of 42.25. And I could get my
calculator out at this point, but I'll actually just
keep using this expression to figure out the total number of lights. So what's the total length of the string? We have to be careful here. A lot of folks would say, "Oh, I figure out the hypothenuse, let me just multiply it that by 10." In fact, my brain almost
did that just now. But we gotta realize
that the entire string is the hypothenuse plus this 2.5. So the whole string length, Write this way. String length is equal to 2.5 plus the square root of 42.25. And then we would just
multiply it that time 10 to get the total number of lights. So now let's actually
get the calculator out. So we have 42.25 and then we would've take the square root of that. Gets us to 6.5 and then
we add the other 2.5, plus 2.5 equals that. That's the total length of string. So the total length of
string is nine meters and there're 10 lights per meter. So the number of lights, number of lights is equal to
nine total meters of string times 10 lights per meter, which would give us 90 lights. Now some of you might debate, if you think really deeply about it, is if you have a light
right at the beginning, if these were kind of
set up like a fence post, then maybe you can argue
that there's one extra light in there, but for the sake of this, I kind of view this as, on average there're 10 lights per meter and so if it's a nine meter string, nine times 10 is 90 lights.