The solution of the differential equation $${{{d^2}y} \over {d{x^2}}} = 0$$ represents

The solution of the differential equation $$\frac{d y}{d x}+\sqrt{\frac{1-y^2}{1-x^2}}=0$$ is

The solution of the differential equation $$x \frac{d y}{d x}=\cot y$$ is

The solution of the differential equation $${\sec ^2}x\tan ydx + {\sec ^2}y\tan xdy = 0$$ is