Equation of vertical lines is x = c
∴ Equation of non-vertical lines is x is not a constant.
Let y = mx + c, is equation of non-vertical line.
Then \(\frac{dy}{dx}=m\)
⇒ \(\frac{d^2y}{dx^2}=0\)
Differential equation of non-vertical lines in xy-plane is \(\frac{d^2y}{dx^2}=0\)
