18/11减括号7/11+3/8括号=18/11-（7/11+3/8）=18/11-7/11-3/8=（18/11-7/11）-3/8=11/11-3/8=8/8-3/8=5/8
18/11-（7/11+3/8）=18/11-7/11-3/8=1-3/8=5/8
