[填空题]
A 31.0-cm-focal-length cueml; )o7p3uf onve19uo-j8m)vrn wk)sg3a yif7q rging lens is 21.0 cm behind a diverging lens. Parallel light strikes the diverging lens. After passing through the converging lens, the light is again parallel. What is the focal length of the diverging lens? m 837n9-ofq jy)ui1wkg rs)va[Hint: first draw a ray diagram.] cm
