Question

A concave mirror is a part of sphere of radius 20 cm, then focal length of concave mirror is:-

Answer 1

$R = 2f \\ here \ \\ : R = radius \\ : f = focal \: length \\ R = 20 \\ f = \frac{20}{2} \\ f = 10cm$
$\huge{\boxed{\red{\mathfrak{10\:cm}}}}$

Answer 2

Radius of curvature= 20 cm.
then focal length (f) will be
1/2×20= 10cm.