Judging from your picture combination, in the description of the topic, it may be that at the same time, B sailed due north at the speed of 10km/h, and after how long, the distance between the two ships was the shortest.

We should sail west.

Then find the distance of MN

So I'm n in the triangle

Just use Pythagorean theorem to find MN.

Suppose two ships are closest after x hours.

Then BN= 10x.

AM=20x，AN = 50- 10x 0 & lt； = x & lt=5

Mn 2 = am 2+(50-10x) 2 = (20 times) 2+(50-10x) 2.

MN= 10* quadratic root number (5x 2- 10x+25)

Let y = (5x 2- 10x+25)

=5[(x- 1)^2+4]

When x= 1, y has a minimum value.

So there is a minimum after 1 hour.