An element reacts with hydrogen to form a compound $A$ which on treatment with water liberates hydrogen gas. The element can be

If $f: R \rightarrow R$ is defined by $f(x) = [\frac{x}{5}]$ for $x \in R$,where $[y]$ denotes the greatest integer not exceeding $y$,then $\{f(x) : |x| < 71\}$ is equal to

For $100\%$ modulation $(AM)$,the useful part of the total power radiated is

$\cos 36^{\circ} - \cos 72^{\circ}$ is equal to

The number of arrangements of the letters of the word $PALANHAR$ in which no two vowels are together and exactly two vowels are at odd places,is

What is formed when $HCOOH$ reacts with concentrated $H_2SO_4$?