=(7/9+4/21-9/21)*6.3=(7/9-5/21)*6.3=4.9-1.5=3.4
7/9*6.3+4/21*6.3-3/7*6.3=4.9+1.2-2.7=3.4
=(7/9+4/21-9/21)*6.3=(7/9-5/21)*6.3=7/9*6.3-5/21*6.3=4.9-1.5=3.4
(7/9+4/21-3/7)乘6.3=7/9x6.3+4/21x6.3-3/7x6.3=4.9+1.2-2.7=3.4