[img]file:///C:\Users\Administrator\AppData\Roaming\Tencent\Users\1014062452\QQ\WinTemp\RichOle\JDI[VE1PTX$4]@2U7`QB_WB.png[/img][img=0,1]file:///C:\Users\Administrator\AppData\Roaming\Tencent\Users\1014062452\QQ\WinTemp\RichOle\JDI[VE1PTX$4]@2U7`QB_WB.png[/img]
不懂这一步的推导过程，求大神指点！