The equation of the curve passing through the point $$\left(0, \frac{\pi}{4}\right)$$ and satisfying the differential equation $$\left(e^x \tan y\right) d x\left.+\left(1+e^x\right) \sec ^2 y\right) d y=0$$ is given by

The solution of the differential equation $$2x\left(\frac{dy}{dx}\right)-y=4$$ represents a family of

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