pm²/(am-b)-pm/a
=apm²/a(am-b)-[(apm²-bpm)/a(am-b)]
=bpm/(a²m-ab)
将m=ab/a²-b代入得：
[ab²p/(a²-b)]/[a³b/(a²b)-(a³b+ab²)/(a²-b)]
=p