请参考http://zhidao.baidu.com/link?url=gV2ereP2CT3uv_YEe_MikQ8U-C7-yokjJqwpME_S3sc186Fnq82vwcJIRHpukoXmkhxx4TFjvp5JeIutsCo7cK
4BC=6CDBC=1.5CD1.5CD+CD=2.5CD=30CD=12BC=1.5*12=18面积=18*4=72
