If $(2, 0)$ is the vertex and the $y$-axis is the directrix of a parabola,then its focus is

The difference between the $n^{\text{th}}$ and $(n+1)^{\text{th}}$ Bohr's radius of a hydrogen atom is equal to the $(n-1)^{\text{th}}$ Bohr's radius. The value of $n$ is:

Which of the following compounds when heated with $CO$ at $150^{\circ} C$ and $500 \ atm$ pressure in the presence of $BF_3$ forms ethyl propionate?

Two particles $P$ and $Q$ start from the origin and execute simple harmonic motion along the $X$-axis with the same amplitude but with periods $3 \ s$ and $6 \ s$,respectively. The ratio of the velocities of $P$ and $Q$ when they meet is

What product is obtained from the reaction of a Grignard reagent with a ketone?

If $\cos (\alpha+\beta)=\frac{4}{5}$,$\sin (\alpha-\beta)=\frac{5}{13}$ and $\alpha, \beta$ are between $0$ and $\frac{\pi}{4}$,then $\tan 2\alpha$ is equal to