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