The differential equation whose solution represents the family $x^2 y=4 \mathrm{e}^x+\mathrm{c}$, where c is an arbitrary constant, is

In hydrogen spectrum, the ratio of wavelengths of the last line of Lyman series and that of the last line of Balmer series is

For a perfectly black body, coefficient of emission is

The potential difference $\left(V_A-V_B\right)$ between the points A and B in the given figure is