In an acute angled triangle,$\cot B \cot C + \cot A \cot C + \cot A \cot B$ is equal to

Which of the following is not an air pollutant?

An electron moves with speed $2 \times 10^5 \ m/s$ along the positive $x$-direction in the presence of a magnetic field of induction $B = (\hat{i} + 4\hat{j} - 3\hat{k}) \ T$. The magnitude of the force experienced by the electron in newtons is (Charge on the electron $= 1.6 \times 10^{-19} \ C$)

The intensity of radiation emitted by the sun has its maximum value at a wavelength of $510 \ nm$ and that emitted by the north star has the maximum value at $350 \ nm$. If these stars behave like black bodies,then the ratio of the surface temperature of the sun and north star is

The two surfaces of a concave lens,made of glass of refractive index $1.5$,have the same radii of curvature $R$. It is now immersed in a medium of refractive index $1.75$. Then the lens:

Magnesium wire is heated in the atmosphere of $CO_2$ because