00:01
When you're given c of x is 4 ,000 plus 4x plus 0 .02x squared plus 0 .001, i can double check 1, 2 ,3, x cubed.
00:19
And i'm just going to do the power rule where its derivative is going to be, well, the derivative of a constant is zero, so i don't have to worry about that.
00:31
But the derivative of a linear piece is the coefficient, and the power rule is moving that exponent in front to multiply, and you subtract one from the exponent.
00:44
So three times 0 .001 would be 0 .003, x to the second power.
00:50
I believe that's all you need for that first question.
00:53
But then the second equation, c prime of 140, well, you're going to plug in 140 and for those two xs.
01:14
And i'm just going to use a calculator because i don't trust my doing this in my head.
01:27
That i got 68 .4.
01:32
And so what that tells me is the cost difference between the 140th and the 141st pair.
01:40
So that would be the third option down.
01:45
And the reason for that is because c prime represents the slope at x equals 140...