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[Lukybear]

A straight railway track and a straight road intersect at right angles. At a given instant, a motor car at 40 km/h and a train at 50 km/h are moving away from the intersection and are 40 and 30 km respectively from the intersection. At what rate is the distance between them changing one hour later? At what rate would the distance between them be changing at that instant if they were traveling towards the intersection.

Dunno this one as they are 3 variables

Help!

 

[~caramello~]

make two equations for the car and train with time variable
let car = 40 + 40t
train = 30+50t

Use pythagoras theorem: distance = sqrt(distance of car + distance of train)
then differentiate with respect to time substituting t = 1 ( after 1 hour)