In an intrinsic semiconductor at room temperature,the number of electrons and holes are:

The solution of $(x^2+y^2) dx = 2xy dy$ is:

In $\triangle ABC$,if $\frac{1}{b+c} + \frac{1}{c+a} = \frac{3}{a+b+c}$,then $C$ is equal to (in $^{\circ}$)

Maximum contribution of $O_2$ is from

The magnifying power of a simple microscope is (when the final image is formed at $D = 25 \ cm$ from the eye):

$A$ pipe closed at one end has a length of $0.8 ~m$. At its open end,a $0.5 ~m$ long uniform string is vibrating in its second harmonic and it resonates with the fundamental frequency of the pipe. If the tension in the wire is $50 ~N$ and the speed of sound is $320 ~m/s$,the mass of the string used is: (in $~g$)