what is shunt?derive the equation of shunt.

Dear Student ,
Shunt is a path which allows almost all current to pass through the path . It is a resistor which is connected in parallel with measuring device. 
Let a shunt is resistance R_s is connected in parallel with the resistance R and a current I flow through R and I_s through R_s .
So , the R_s  will be given as
$$\begin{gathered} I_s*R_s=I*R \\ So, R_s=\frac{I*R}{I_s} \end{gathered}$$
Hope this helps you dear .

Regards