I asked this just before 5 minutes but the answer was given 1/v-1/u= 1/f but the formulae of convex mirror is 1/v+1/u=1/f

1.  A convex lens has a focal length of 10 cm. At what distance from the lens should the object be placed so that it forms a real and inverted image 20 cm. away from the lens? What would be the size of the image formed if the object is 2 cm high? With the help of a ray diagram show the formation of the image by the lens in this case? 

Dear Student ,
Kindly read the question carefully here in this particular question lens is used not a mirror .
So here the theorem or the equation used by the expert is absolutely correct and if there is some doubt regarding the answer still persists then you can give the screenshot of the question along with the answer given .We will check it as soon as possible and rectify it .
But here the answer given by the expert is correct .

  • 1
1/f =1/v -1/u
1/10 = 1/20 - 1/u
1/20 - 1/10 = 1/u
-1/20 = 1/ u
u = - 20cm

hi/ho = v/u
hi/2= 20 / -20
hi = -2 cm

  • 2
f=10cm 2f=20cm V=20cm 1/f=1/v - 1/u 1/10=1/20 -1/u 1/u=1/20 - 1/10 1/u=1/20 - 2/20 1/u=-1/20 u= -20cm
  • 1
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