1. The problem statement, all variables and given/known data Car A is traveling north on Highway 16 and car B is traveling west on Highway 83. Each car is approaching the intersection of these highways. At a certain moment, car A is 0.3 km from the intersection and traveling at 90km/h while car B is 0.4km from the intersection and traveling at 80km/h. How fast is the distance between the cars changing at that moment? Attempt : D = sqrt(x^2 + y ^ 2) Let C = sqrt(x^2 + y^2); d(D) = dD/dx dx + dD/dy dy = x/C dx + y/C dy we are given x = 0.3, y = 0.4 , dx = 90 and dy = 80, so d(D) = dx/6 + 8dy/5; Now just plug in dx and y , so the answer I got is 143 . Can you check this?