！
8/9×[3/4-(7/16-1/4)]=8/9×[3/4-7/16+1/4]=8/9×[1-7/16]=8/9×1-8/9×7/16=8/9-7/18=16/18-7/18=1/2
=8/9x[3/4-11/16]=8/9x1/16=8/9
