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CCSS.Math: , , ,

- [Instructor] We're
told that Rodrigo watches a helicopter take off from a platform. The height of the helicopter,
in meters above the ground, t minutes after takeoff is modeled by, and we see this function right over here. Rodrigo wants to know when the helicopter will land on the ground. So pause this video and see
if you can figure that out. All right, now let's
think about this together. So let's just imagine
actually what the graph of this function looks like. And it'll also help us
imagine what's going on with the helicopter. So our horizontal axis,
this is t, time in minutes, and then our vertical axis is height. So height as a function of time. And maybe I just write it like this. I'll just write height, and this is given in
meters above the ground. Now I don't know exactly
what the graph looks like, but given that I have
a negative coefficient on my quadratic term, I know that it is a downward
opening parabola like that. And it says that the helicopter
takes off of a platform. So however high the platform is, then it takes off and it's
going to do something like this. I don't know exactly what
the graph looks like, but probably something like this. Now, if they asked us
what is the highest point of the helicopter, and at
what time does it happen, then we'd wanna figure
out what the vertex is of this parabola. But that's not what they're asking. They're asking when does the
helicopter land on the ground? That's this time right over here. So if we wanted to find the vertex, we would wanna put this into vertex form, but here we wanna figure out when does that function equal zero. We want to find a zero of this
quadratic right over here. So the best way that I
can think about doing it is try to factor it, try to set them this thing equal to zero, and then factor it and
then see what t values make that equal to zero. So let me do that. So I say negative three t
squared plus 24t plus 60, remember we care when our height is equal to zero, equal zero. So let's see maybe the
first thing I would do just to simplify the second
degree term a little bit. Let's just divide both
sides by negative three. If we did that, this would become t squared
24 divided by negative three is negative eight, negative eight t. 60 divided by negative
three is negative 20, and then zero divided by negative three is of course still zero. And now can I think of
two numbers whose product is negative 20? So they would have different signs in order to get a negative product, and who sum is negative eight. So let's see what about
negative 10 and two, and that scene seems to work. So I could write this as t
minus 10, times t plus two is equal to zero. And so in order to make
this expression equals zero, either one of these
could be equal to zero. So either t minus 10 is equal to zero, or t plus two is equal to zero. And of course on the left here, I can add 10 to both sides. So either t equals 10, or I could subtract two
from both sides here, t is equal to negative two. So there's two places where the function is equal to zero. One at time t equals negative two, and one at time t is equal to 10. Now we're assuming we're
dealing with positive time here. We don't know what the
helicopter was doing before the takeoff. So we wouldn't really think about this. So what we really care about is that t is equal to 10 minutes. That's when the helicopter
is right over there. And actually we know at t equals zero, these two terms become zero. We know it takes off at 60 meters. It goes up. If we figured out the vertex, we would know how high it went, but then it starts going back down, and in 10 minutes after takeoff, it is back at zero, back on the ground.