1. a+b=b+a 2.(a+b)+c=a+(b+c) 3.a+0=a 4.a+(-a)=0 5.(2a)=(A) 6.(+a=+a 7. n(a+b=na+nb 三1. 2. ( ) ( ) 3. 4. ( ) 5. ( ) ( ) 6. ( ) 7. ( ) 8. 1 + = + + + = + + + = + − = = + = + + = + = a b b a a b c a b c a 0 a a a 0 a a a a a a b a b a a