2/3 * 4/7 * 3/4=2/3 * 3/4 * 4/7=1/2 * 4/7=2/7
(18 + 2/7)* 7/9=18* 7/9 + 2/7* 7/9 = 14+ 2/9=14又2/9
15/16 *（3/5 + 1/3）=15/16 * 3/5 + 15/16 * 1/3 =9/16 + 5/16 =14/16 =7/8
14/15 * 7/17 * 5/28 * 17 =（14/15 * 5/28）*（ 7/17 * 17）=1/6 * 7=7/6
其实主要就是运用乘法交换率和乘法分配率,