In a series of $2n$ observations,half of them are equal to $a$ and the remaining half are equal to $-a$. If the standard deviation of the observations is $2$,then $|a|$ is equal to:

The maximum tension a rope can withstand is $60 ~kg-wt$. The ratio of maximum acceleration with which two boys of masses $20 ~kg$ and $30 ~kg$ can climb up the rope at the same time is

The symbiotic association between algal and fungal components is known as ......

The apparent coefficient of expansion of a liquid when heated in a copper vessel is $C$ and when heated in a silver vessel is $S$. If $A$ is the linear coefficient of expansion of copper,then the linear coefficient of expansion of silver is

If $x^y=y^x$, then $x(x-y \log x) \frac{d y}{d x}$ is equal to :

If the sum of any two roots of the equation $x^3+p x^2+q x+r=0$ is zero,then