# Math Help - fast is marginal cost changing

1. ## fast is marginal cost changing

If C(x)=2.0x^2+9x+200 is a cost function, how fast is marginal cost changing when x=100?

I took the derivative and got 4x+9 and plugged the 100 in and got 409. However, I was told that in order to find how fast the marginal cost is changing i have to do the second derivative. I am not sure which answer is right.
Can someone explain to me whether i take have to find the second derivative in order to solve this questions.

Please and thank you

2. Originally Posted by happy8588
If C(x)=2.0x^2+9x+200 is a cost function, how fast is marginal cost changing when x=100?

I took the derivative and got 4x+9 and plugged the 100 in and got 409. However, I was told that in order to find how fast the marginal cost is changing i have to do the second derivative. I am not sure which answer is right.
Can someone explain to me whether i take have to find the second derivative in order to solve this questions.

Please and thank you
Yes, it should be the second derivative. the first derivative is the marginal cost. to find the rate at which that is changing, you would need to differentiate it (take another derivative).