00:01
So in this question, we have two straight highways, one running west to east, and the other running north to south, meeting at an intersection.
00:10
We have car a, which is on the east -west highway, 300 meters east of the intersection.
00:21
And it's traveling at 90 kilometers per hour.
00:25
So let's see.
00:27
Here we'll say is the intersection.
00:30
And we have car a east of the intersection by 300 meters.
00:41
Now notice that there are 1 ,000 meters in a kilometer, and everything else in this question is in kilometers.
00:50
So i'm going to say that this 300 meters, i'm going to convert that to kilometers.
00:55
I'm going to say this is 0 .3 kilometers.
00:59
Now, this distance from the intersection to car a, it's changing.
01:05
So i'm going to call that guy x.
01:08
When they say that the car is traveling east at 90 kilometers per hour, what are they giving us? they are giving us dxdt.
01:20
So my dxdt this time is 90 kilometers per hour.
01:28
Now, car b is on the north -south highway.
01:31
And it's 400 meters south of the intersection.
01:37
So car b is down here.
01:39
Now, the distance from the intersection to car b, that's changing.
01:45
So i'm going to call that y.
01:48
And that 400 meters, i'm going to think of that as 0 .4 kilometers.
01:57
And car b is traveling north at 85 kilometers per hour.
02:01
What are they getting us there? they're giving us dydt.
02:08
Now, dydt is not positive 85, but it's negative 85 because y is decreasing.
02:19
So my dydt here is negative 85 kilometers per hour...