[填空题]
An achromatic lens is made of two vep 2akh3luzv52 g;)qk3zxec5 zry thinna8bkyj /x )z.w ap-2v lenses, placed in contact, that have focal lengths/wyb8an) .2kpv-jz xa of $f_1=-28\mathrm{~cm}$ and $f_2=+23\mathrm{~$cm$}.$
(a) Is the combination converging or diverging?
(b) What is the net focal length? m