Ò»¡¢Ñ¡ÔñÌ⣺£¨±¾´óÌâ¹²12СÌ⣬ÿСÌâ5·Ö£¬¹²60·Ö£©
1¡¢ÒÑÖª¼¯ºÏ Ôò =£¨ £©
A£® B£® C£® D£®
2¡¢ÔÚ¡÷ABCÖУ¬ÒÑÖªb2£½acÇÒc£½2a£¬Ôòcos BµÈÓÚ(¡¡¡¡)
A£® B£® C£® D£®