Question

the focal length of convex lens is f .an object is placed at a distance x from its first focus then findthe ratio of size of the real image to that of the object

Answer 1

Here is the solution. check it.

Answer 2

The  ratio of the size of a real image to that of an object is f:x.

Focal length of the convex lens = f (Given)

Distance of the object = x  (Given)

According to the lens formula -  

1/v + 1/f+x = 1/f

v = f(f+x)/x

= Ratio of size/magnification

= f(f+x)/x/ f+x

= f/x

Therefore, the  ratio of the size of a real image to that of an object is f:x.