Integral Calc and Economics / Differential Calc and Biology

Hi there~

I am trying to come up with an example of applying Integral Calculus to Economics, and was thinking something along the lines of finding maximal cost to find marginal cost and revenue, but it's been a while since I've dove into Econ.

Also, in regards to differential calculus, I need to show an application present in Biology, would growth rate apply here? Any advice, suggestions and guidance would be greatly appreciated! Thank you!