12/7×1/9×5/9×3/7=12/7×1/9×5/3×1/7=4/7×1/9×5/1×1/7=20/441
简便计算12/7x1/9+5/9x3/7=12/7x1/9+1/9x15/7=(12/7+15/7)x1/9=27/7x1/9=3/7
12/7x1/9+5/9x3/7=12/7x1/9+1/9x15/7=(12/7+15/7)x1/9=27/7x1/9=3/7