A road perpendicular to a highway leads to a farmhouse located 6 mile away. An automobile traveling on the highway passes through this intersection at a speed of 75mph.

How fast is the distance between the automobile and the farmhouse increasing when the automobile is 10

miles past the intersection of the highway and the road?

2 answers

if the car has gone x miles, the distance z to the farmhouse is

z^2 = x^2 + 36

so, find z(10) and then

z dz/dt = x dx/dt

and plug in your numbers.
h = highway distance from intersection

d = auto-to-farmhouse distance

by Pythagoras ... d^2 = h^2 + 6^2

differentiating implicitly
... 2 d dd/dt = 2 h dh/dt

dd/dt = h / d * dh/dt