00:01
Here we have two formulas, one for revenue and one for cost.
00:05
R is equal to 50x minus 0 .4x squared and c is equal to 5x plus 15.
00:12
What we want to do is find the rate of change with respect to time when dxdt is equal to 10 and x is equal to 40.
00:22
And we're also going to find the change in cost with respect to time when dxdt is 10 and x is 40.
00:29
So let's go ahead and take the derivative of our revenue function this is going to be 50 dx d t minus 0 .8x d .t.
00:46
So drdt, we're going to plug in the things that we do know, 50 times 10 minus 0 .8 times 40 times dxdt, which is 10.
01:03
We're going to go ahead and calculate that all out.
01:06
We're going to end up with 180.
01:09
Next, we want to find dcdt.
01:16
So this is going to be 5 dxdt.
01:22
So dcdt is going to be equal to 5 times 10...