if a convex lens of focal length 20cm and a concave lens of focal length 30cm kept in contact with each other first what will be the focal length of the combination second what will be the power of the combination third what will be the behaviour of the combination

Hi, your answer is not correct. this is a cumulative  

Answerthe energy required is 1.60 × 10⁻¹⁸ j, to three significant figures.explanationwe define electric potential as the ratio electric potential energy to the charge,    then the following gives us the potential difference,      the formula  may be on your formula sheet.the change in electric potential energy would be the energy that would needed to move it from point a to point b. solving for that, we get    using q = 8.00  × 10⁻¹⁹ c and  δv = 2.00 v, we get    the energy required is 1.60 × 10⁻¹⁸ j, to three significant figures.