Chứng minh rằng x,y ∈ Q
a)|x+y |$\leq$ |x|+|y|
b) |x-y| $\geq$ |x|-|y|
Chứng minh rằng x,y ∈ Q a)|x+y |$\leq$ |x|+|y| b) |x-y| $\geq$ |x|-|y|
By Valentina
By Valentina
Chứng minh rằng x,y ∈ Q
a)|x+y |$\leq$ |x|+|y|
b) |x-y| $\geq$ |x|-|y|
Giải thích các bước giải:
a) $\forall x,y \in Q : $
$x\leq \left | x \right |,-x\leq \left | x \right |$
$y\leq \left | y \right |,-y\leq \left | y \right |$
$\Rightarrow x+y\leq \left | x \right |+\left | y \right |,-x-y\leq \left | x \right |+\left | y \right |$
$\Rightarrow x+y\geq -\left ( \left | x \right |+\left | y \right | \right )$
$\Rightarrow -\left ( \left | x \right |+\left | y \right | \right )\leq x+y\leq \left | x \right |+\left | y \right |$
$\Rightarrow \left | x+y \right |\leq \left | x \right |+\left | y \right |$
b) Theo câu a :
$\left | x-y \right |+\left | y \right |\geq \left | x-y+y \right |= \left | x \right |$
$\Rightarrow \left | x-y \right |\geq \left | x \right |-\left | y \right |$