AC＞AB.DC＞DB.DC上取E；使DE＝DB.连接AE,PE,则AE=AB.PE=PB.
设AE,PC交于O,有：PO+OE＞OE=PB.AO+OC＞AC.
两式相加得：PO+OE+AO+OC＝PC+AE＝PC+AB＞AC+BP.