Triangles have geometric relations.

s^2=x^2+h^2

Take the derivative of t on both sides of the equation.

d(s^2)/dt=d(x^2+h^2)/dt

2s(ds/dt)=2x(dx/dt)

s*v0=x*v

v=v0(s/x)=v0/cosa

This is a mathematical reasoning method, which can be solved more conveniently.

Sometimes, problems can't be judged intuitively. It is very difficult to seek the truth from the combined speed and the divided speed.

If we have to find out which is the combined speed and which is the divided speed, we can prove it wrong.

As shown in the figure below, if a small M moves with the rope, the horizontal component velocity vM=V0*cosa of M is every problem, but the movement of the ship and M is not the same thing. So the ship speed v=v0*cosa must be wrong.

V=v0/cosa is the combination speed, and v0 is the segmentation speed. You can barely explain it with the above picture.