math
posted by lo .
A road perpendicular to a highway leads to a farmhouse located 8 mile away.An automobile travels past the farmhouse at a speed of 80 mph.
How fast is the distance between the automobile and the farmhouse increasing when the automobile is 7 miles past the intersection of the highway and the road?
The distance between the automobile and the farmhouse is increasing at a rate of ___________ mph.

Let the distance from the sideroad be x miles, let the distance between the car and the house be y miles.
You have a rightangled triangle where
x^2 + 8^2 = y^2
then 2x dx/dt = 2y dy/dt (1)
when x=7, y^2 = 49+64 = 113
so when x=7, y= √113 and dx/dt = 80
sub these values into equation (1) and solve for dy/dt 
52.68 mph