申論題

2. An experiment detects that a particle at (1,0) (on the xy-plane) is moving towards the north at the speed 3 meters per minute. At the same time another particle at (-2,1) is moving towards the east at the speed 4 meters per minute.

(b) (10%) Suppose at t= t₀ the two particles are closest to each other. Then t₀=_____