Which of the following statements are true? select all that apply.
1. for all real numbers a and b, a+b=b+a
2. for all real numbers a and b, ab=ba
3. for all real numbers a and b, a-b=b-a
4. for all real numbers a, b and c, (a+b) + c = a + (b+c)
5. for all real numbers a, b and c, (a-b) - c = a - (b-c)
6. for all real numbers a, b and c, a(b+c)= ab+ac