If $$x, y, z$$ are in A.P. and $$\tan ^{-1} x, \tan ^{-1} y$$ and $$\tan ^{-1} z$$ are also in A.P., then

Mean and variance of six observations are 8 and 16 respectively. If each observation is multiplied by 3, then new variance of the resulting observations is

The differential equation of all circles, passing through the origin and having their centres on the $$\mathrm{X}$$-axis, is

$$\int_\limits0^1 \cos ^{-1} x d x=$$