Two cars are traveling along perpendicular roads, car a at 40 mi/hr, car b at 60 mi/hr. at noon, when car a reaches the intersection, car b is 90 mi away, and moving toward it. at 1 p. m. the distance between the cars is changing, in miles per hour, at the rate of:

negative sign indicates that the distance is decreasing with time

Explanation:

Let at any time t after noon that is 12 p.m.  

distance traveled by car A = 40t

distance traveled by car B = 90-60t

then distance between the two cars at time t

............1

also, at time 1 p.m.

distance

D=50 Km

differentiating equation 1 w.r.t. t we get

put t= 1 and D= 50 we get

heats surface water up