# Thread: Related Rates question 2

1. ## Related Rates question 2

Two cyclists depart at the same time from a starting point along routes making an angle of 60 degrees with each other. The first is travelling at 15km/h, while the second is moving at 20km/h. How fast are the two cyclists moving apart after 2 hours?

2. Originally Posted by math123456
Two cyclists depart at the same time from a starting point along routes making an angle of 60 degrees with each other. The first is travelling at 15km/h, while the second is moving at 20km/h. How fast are the two cyclists moving apart after 2 hours?
You know the lengths of the two sides of a triangle and the angle between the two sides so you can use the cosine law, $c^2= a^2+ b^2- 2ab cos(C)$ to get a formula for the third side, the distance between them. Differentiate to get a formula connecting the rates.