18该怎么写步骤呢？∵AF1－AF2＝6BF1－BF2＝6∴AF1＋BF1－﹙AF2＋BF2﹚＝12∴AF1＋BF1＝12＋﹙AF2＋BF2﹚＝12＋3＝15∴△abf2(f2)(f2为右焦点)的周长是AF1＋BF1＋AB＝15＋3＝18