Question

when an object placed at 30 cm in fornt of concave mirror image of the same size is formed what is the focal length of the image​

Answer 1

given same sized image is formed.

I,e u=-30cm, v= -30cm

that means the object is placed at the center of curvature.

hence the focal length is -30/2= -15 cm

the minus sign just suggests that the focal length is on the left side of the mirror as per the standard convention.

F= -15cm