In a concave mirror, an object is placed at a distance x from the focus and the image is formed at a distance y from the focus. What is the focal length of the mirror?

$(xy)^{\frac{1}{2}}$ $\frac{(x+y)}{2}$ $xy^{-1}$ $xy$

$(xy)^{\frac{1}{2}}$

From Newtons mirror formula $ f = \sqrt{xy}$