1.

A convex lens of focal length `10cm` and concave lens of focal length `20 cm` are kept `5 cm` apart. The focal length of the equivalent lens isA. `(120)/(3)cm`B. `18cm`C. `30 cm`D. `(40)/(3)`

Answer» Correct Answer - D
`(1)/(F)=(1)/(10)+(1)/(-20)-(5)/(10(-20))`
`(1)/(F)=(1)/(10)-(1)/(20)+(5)/(200)`
`(1)/(F)=(20-10+5)/(200)`or `F=(200)/(15)cm=(40)/(3)cm`


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