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Two thin lenses of focal length f1 and f...

Two thin lenses of focal length `f_1` and `f_2` are placed in contant .The focal length of the composite lens will be

A

`(f_1+f_2)//2`

B

`(f_1+f_2)//f_1f_2`

C

`sqrt(f_1//f_2)`

D

`f_1f_2//(f_1+f_2)`.

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