
Question on trigo?
Hi I really have no idea which catergory this question falls into, especially so when I don't even know how to start off the question..
If Person A is 20Km north of Person B at 5pm. Person A walks south at a rate of 7Km/h and Person B walks west at a rate of 9Km/h. What time do these two peopole stop approaching each other and instead, drift further apart?
How do I start and what formula should I use? I'm really clueless. Is it under bearings..?
Thank you so much, really!

Re: Question on trigo?
let person B start at the origin,
person A's position as a function of time, , in hrs ...
person B's position ...
distance, ,between the two at any time in hrs ...
simplify, then determine when changes sign from negative to positive.

Re: Question on trigo?
This is how I would set it up:
Let north be the positive yaxis and west be the positive xaxis. Distances are in km and time is in hrs.
Person A's position at time t is:
Person B's position at time t is:
Let be the distance separating the two. Hence:
We see this is a parabola opening upward, so all you need to do is find the axis of symmetry to find the time when their distance is a minimum.
Recall the axis of symmetry for the parabola is .