Question

Two cars start moving from the same point. one travels south at 28 mi/h and the other travels west at 21 mi/h. at what rate is the distance between the cars increasing two hours later?

Answer 1

d = distance
d^2 = x^2 + y^2
dd' = 2xx' + 2yy' Taking the derivative wrt time
d' = (2xx' + 2yy')/d
d' = (42x21 +56x28)/70 = 35 mph