1, (6+6+6)÷6=3, calculated in brackets first, 6+6+6= 18, 18÷6=3.

2, (6×6)÷(6+6)=3, calculated in brackets first, 6×6=36, 6+6= 12, 36÷ 12=3.

3, 6 ÷ [(6+6) ÷ 6] = 3, calculated in brackets first, 6+6= 12, and then calculated in brackets:12 ÷ 6 = 2,6 ÷ 2 = 3.

Law of addition:

One-digit addition: When two one-digit numbers are added, the sum can be obtained directly by counting.

The result of adding two one-digit numbers is usually compiled into an addition table.

Multi-digit addition: the addition of numbers on the same digit. One number adds up to ten, then go to the previous number and enter one.

Multiple digits plus multiple digits, you can first write two multiple digits as the sum of different counting units.

Then, according to the arithmetic of addition and the law of one-digit addition, the numbers with the same counting unit are added respectively.