If x and y be the distances of the object and image formed by a concave mirror from its focus and f be the focal length then

$xf = y^2$ $xy = f^2$ $x/y = f$ $x/y = f^2$

$xy = f^2$

According to Newton's formula $xy = f^2$. Note that $m=\frac{f}{f-u}=\frac{f-v}{f}$ or $\frac{f}{x}=\frac{y}{x}⇒xy=f^2$