I found this question on Yahoo! Answers, and I am stumped (however, I would like to know the answer):

Let f(x,y) be a continuous function in R2. For t>0 let R_t = [0,t] x [0,2t] = { (x,y) | 0<=x<=t, 0<=y<=2t } and define F(t) integral integral R_t of f(x,y)dA Use Leibniz Rule to compute the derivative dF/dt.