How can the power of the combination expressed, if two lenses are equibiconcave and separated by a distance?

Explanation

`(1)/(f)= (1)/(-f_(1))+(1)/(-f_(2))-(d)/((f_(1))(f_(2)))`
i.e., `(1)/(-f)= (1)/(f_(1))+(1)/(f_(2))+(d)/(f_(1)f_(2))`