The two lenses of an achromatic doublet should have:

The solution of $\frac{dx}{dy} + \frac{x}{y} = x^2$ is:

If the imaginary part of $\frac{2z+1}{iz+1}$ is $-2$,then the locus of the point representing $z$ in the Argand plane is

$A$ transformer having an efficiency of $90\%$ is working on a $200\, V$ and $3\, kW$ power supply. If the current in the secondary coil is $6\, A$,the voltage across the secondary coil and the current in the primary coil respectively are:

In a transistor,if $\frac{I_C}{I_E} = \alpha$ and $\frac{I_C}{I_B} = \beta$. If $\alpha$ varies between $\frac{20}{21}$ and $\frac{100}{101}$,then the value of $\beta$ lies between:

The Effective Atomic Number $(EAN)$ of $Co(CO)_4$ is $35$,hence it is less stable. It attains stability by: