含有绝对值的不等式 这一节中.书上有:“—( |a| + |b| ) ≤ a + b ≤ |a| + |b|即 |a + b| ≤ |a| + |b| ”怎么会从“—( |a| + |b| ) ≤ a + b ≤ |a| + |b|”得出或看出“|a + b| ≤ |a| + |b|呢?

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含有绝对值的不等式 这一节中.书上有:“—( |a| + |b| ) ≤ a + b ≤ |a| + |b|即 |a + b| ≤ |a| + |b| ”怎么会从“—( |a| + |b| ) ≤ a + b ≤ |a| + |b|”得出或看出“|a + b| ≤ |a| + |b|呢?

含有绝对值的不等式 这一节中.书上有:“—( |a| + |b| ) ≤ a + b ≤ |a| + |b|即 |a + b| ≤ |a| + |b| ”怎么会从“—( |a| + |b| ) ≤ a + b ≤ |a| + |b|”得出或看出“|a + b| ≤ |a| + |b|呢?
含有绝对值的不等式 这一节中.
书上有:
“—( |a| + |b| ) ≤ a + b ≤ |a| + |b|
即 |a + b| ≤ |a| + |b| ”
怎么会从“—( |a| + |b| ) ≤ a + b ≤ |a| + |b|”得出或看出
“|a + b| ≤ |a| + |b|呢?

含有绝对值的不等式 这一节中.书上有:“—( |a| + |b| ) ≤ a + b ≤ |a| + |b|即 |a + b| ≤ |a| + |b| ”怎么会从“—( |a| + |b| ) ≤ a + b ≤ |a| + |b|”得出或看出“|a + b| ≤ |a| + |b|呢?
从“—( |a| + |b| ) ≤ a + b ≤ |a| + |b|”不能得出或看出“|a + b| ≤ |a| + |b|”
a,b同号时,|a + b| = |a| + |b|;
a,b异号时,|a + b| <|a| + |b|.