In which plant was the importance of day length for flowering first observed?

Differentiate the function with respect to $x$: $\sec (\tan (\sqrt{x}))$

$A$ body of mass $200\,g$ is tied to a spring of spring constant $12.5\,N/m$,while the other end of the spring is fixed at point $O$. If the body moves about $O$ in a circular path on a smooth horizontal surface with a constant angular speed of $5\,rad/s$,then the ratio of the extension in the spring to its natural length will be:

$\tan 9^{\circ}-\tan 27^{\circ}-\tan 63^{\circ}+\tan 81^{\circ} = $

For skywave propagation of a $10\, MHz$ signal,what should be the minimum electron density in the ionosphere?