加法结合律:a+b+c=a+(b+c) 例题:++ =+(+) =+1 =1 减法的连减:a-b-c=a-(b+c) 例题:2-- =2-(+) =2-1 =1 减法的连减:a-(b+c)=a-b-c 例题:-(+) =-- =1- = a-b+c=a+c-b 例题:-+ =+- =1- = a-(b-c)=a-b+c 例题:2-(-) =2-+ =2+ =2 a-b+c-d=(a+c)-(b+d) 例题:-+- =(+)-(+) =-1 =