1, the principle of obtaining absolute value sign is: if it is greater than or equal to 0, the sign of absolute value will be removed directly; If it is less than 0, the absolute value sign will be deleted and a negative sign will be added before the number. That is, the absolute value of a positive number is itself, and the absolute value of a negative number is its inverse.

2. For the shape ︱ A ︱: (1) When A >: 0, ︱ a ︱a︱= a；; (2) when a=0-A = 0; (3) When a0, ︱ A+B ︱ = A+B; (2) when a+b=0, ︱ a+b ︱ = 0; (3) when a+b