[填空题]
A 31.0-cm-focal-length convergfxek.9 r. aw)i8/spekqr(;4y xnxbn*vnr +gnla k6k; . ex29fing lens is 21.0 cm behind a diverging lens. Parallel light strikes the diverging lens. After passing throug8ek9q;.;v(*b6a24slp+exn kfk/gr nrn xxnyh the converging lens, the light is again parallel. What is the focal length of the diverging lens? [Hint: first draw a ray diagram.] cm
