1、设留下水银x(cm).
P1=75cmHg V1=75-48=27cm
P2=(75-x)cmHg V2=75-x(cm)
P1V1=P2V2
(75-x)^2=75*27
x=30cm
2、（1）P1=71cmHg,V1=19cm,V2=18cm
P1V1=P2V2 P2=74.94cmHg
需注入水银1+4-(75-74.94)=4.94cm
(2)P1=79cmHg,V1=19cm,V2=18cm
P1V1=P2V2 P2=83.39cmHg
需注入水银1+(83.39-75)-3=6.39cm
所以需注入水银4.94cm或6.39cm