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A convex lens of focal length f forms an...

A convex lens of focal length f forms an image which is 1/3 times the size of the object. Then, the distance of object from the lends is

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Image is real so `m = (v)/(u) = -(1)/(n) implies v = -(u)/(n)`
from lens formula `(1)/(v) - (1)/(u) = (1)/(f) implies (1)/(-(u)/(n)) - (1)/(u) = (1)/(f) implies (-(n+1))/(u) = (1)/(f) implies u = -f(1+n)`
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