tanC=3√7,cosC＝1/8,a·b＝-5/2＝-|a||b|/8.∴|a||b|＝20a+b+c＝0,∴c²＝（a+b）²＝a²+b²+2a·b＝a²+b²-5.|a|+|b|=9,a²+b²＝（|a|+|b|）²-|a||b|＝41.c²＝36.|c...