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A thin convex lens from a real Image of ...

A thin convex lens from a real Image of a certain object`'p'`times it size .The size of real image become`'q'`times that of object when the lens is moved nearer to the object by a distance'a'fin dfocal length of the lens?

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The correct Answer is:
`(apq)/((q-p)`
From `1`'st Image `-p=(f)/(f-u)rArr pu-pf=f`
`pu=(p+1)f rArru=(p+1)/(p)f……(1)`
from`2'nd`image `-q=(f)/(-(u-a)+f)`
`qu-qa-qf=f rArr qu=(q+1)f+qa…..(2)`
`q((p+1))/(p)f=(q+1)f+qa rArr qpf+qf=pqf+pf+pqa`
`f(q-p)=pqa`
`f=(pqa)/(q-p)`
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