a.
$\left | 39 + 28 \right | = \left | 67 \right | = 67$
$\left | 39 \right | + \left | 28 \right | = 39 + 28 = 67$
$\Rightarrow \left | 39 + 28 \right | = \left | 39 \right | + \left | 28 \right |$
b.
$\left | -206 + (-35) \right | = \left | -241 \right | = 241$
$\left | -206 \right | + \left | -35 \right | = 206 + 35 = 241$
$\Rightarrow \left | -206 + (-35) \right | = \left | -206 \right | + \left | -35 \right |$
Nhận xét:
$\left | a + b \right | = \left | a \right | + \left | b \right |$ với $a, b \in \mathbb{Z}$
