Solving the Diff. Eq: dy/dx= -x/y

[minia2353]

can someone please help me solve this problem?

let C be a flow line for F(x,y) = -yi+xj, and let (x,y) be a point on C for which y is not 0.
Show that the flow lines satisfy the differential equation
dy/dx= -x/y.

 

[radou]

Start with the fact that dx / F1 = dy / F2, where F = F1 i + F2 j.

 

[minia2353]

I really don't understand...
can you explain the step more thoroughly??