Integral or derivative?

Too simple. I'm using nonlinear differential equations every day to calculate if I still like my wife enough to stay married to her.

I take the integral of how good she has been to me so far and multiply it with how good she is today, and add the square root of how much nicer she is to me today than yesterday. Then I take the absolute value of the whole thing. If it's a positive value, she stays my wife for another day.