What is the difference of the focal distances of any point on the hyperbola?

In case of hyperbola difference between two focal points from any point $P\left(x_1,y_1\right)$ of the hyperbola having eccentricity $=e$ is equal to the length of transverse axis.

i.e., $S^{\mathrm{\prime }}P-SP=\left(e{x}_1+a\right)-\left(e{x}_1-a\right)$

[ where S 'and S are two focal points= 2a]