In a $\triangle ABC$,if $\frac{\cos A}{a} = \frac{\cos B}{b} = \frac{\cos C}{c}$,then $\triangle ABC$ is

$A$ pipe closed at one end has a length of $0.8 ~m$. At its open end,a $0.5 ~m$ long uniform string is vibrating in its second harmonic and it resonates with the fundamental frequency of the pipe. If the tension in the wire is $50 ~N$ and the speed of sound is $320 ~m/s$,the mass of the string used is: (in $~g$)

If force $F$ is increasing with time and at $t=0, F=0$,where will slipping first start?

The number of elements present in the fourth period is

Let $A = \{x \in R, x \neq 0, -4 \leq x \leq 4\}$ and $f: A \rightarrow R$ be defined by $f(x) = \frac{|x|}{x}$ for $x \in A$. Then,the range of $f$ is