If the marginal costs of 1,2,and 3 hours of talking on the phone are $50,$75,and $105,then the total costs are
A) $50,$150,and $315.
B) $50,$41.67,and $115.
C) $50,125,and $230.
D) $50,$175,and $405.
E) impossible to calculate with information given.

Question

Quizplus · Accepted Answer

The total cost is the integral of the marginal cost function. 
So, the total cost of talking on the phone for 1 hour is $\int_0^1 50dx=50.$ 
The total cost of talking on the phone for 2 hours is $\int_0^2 75dx=125.$ 
The total cost of talking on the phone for 3 hours is $\int_0^3 105dx=315.$ 
Therefore, the best choice is C.

If the Marginal Costs of 1,2,and 3 Hours of Talking