00:02
So in this problem, we're given that the price and quantity of a certain product obey this relationship.
00:10
That's x is the quantity and p is the price.
00:18
So x is minus 20p plus 500, where p is greater than 0 and less than or equal to 25.
00:31
We're first asked to express the revenue as a function of x.
00:34
So the revenue r as a function of x is price times quantity.
00:48
So i need the price out of this function up here, out of our given function as a function of x, don't i? so move the 500 other side, that's x minus 500, and then divide everything by the minus 20 for my price.
01:08
Take all this times x, the quantity.
01:11
Okay, so that means i have minus 1 over 20, x times x, that's x squared.
01:18
And then minus 500 over minus 20.
01:21
That's 25 in it, positive 25.
01:27
Okay, so there's the revenue as a function of x.
01:34
So next, i'm asked what's the revenue of 20 units are sold? so the revenue of 20 units is minus 1 over 20 times 20 squared, plus 20.
01:49
25 times 20.
01:53
Well, this dividing by 20 right here knocks out one of those 20s.
02:00
So i have minus 20 plus 25 times 20 is 500.
02:08
So this is $480.
02:15
Okay.
02:18
C says what quantity x maximizes revenue and what is that maximum revenue? so there's two ways, at least two ways to do this.
02:26
One is to take the derivative of, our revenue equation and set it equal to zero to find it.
02:33
The other way is to graph it.
02:35
So let's do the derivative first.
02:41
So i bring the two down off the exponent.
02:44
So that gives me minus 1 over 10 times x.
02:50
2 minus 1 is 1.
02:53
That's how we do the derivative.
02:55
And then plus 25.
02:57
So that x has an exponent of 1 on it.
03:00
So bring the 1 down.
03:01
So this is 25.
03:03
And then the x -1 will be 1 minus 1, which is 0.
03:09
Set this equal to 0 now.
03:15
And so, this is minus 1 over 10x is equal to minus 25, which multiply both sides by minus 10.
03:25
So, x is 250.
03:30
So there's our quantity, right there.
03:34
And then the revenue, at 250.
03:39
It's minus 1 over 20 times 250 squared plus 25 times 250.
03:51
Okay.
03:55
So 250 squared.
04:02
Putting it there, let me put it here.
04:05
It's minus 1 over 20 times 6 ,500 plus 6 ,250...