kOX=5/5=1 ;kOY=2/1=2
设两直线的夹角为θ
tanθ=(kOX-kOY)/(1-kOX*kOY) (次式是直线OY逆时针转到直线OX)
=(1-1/2)/(1-1/2)
=1
θ=π/4
不懂问我!