(x+y)²=x²+y²+2xy=4+2xy≥0
(x-y)²=x²+y²-2xy=4-2xy≥0
所以-2≤xy≤2
|x-y|=√(x-y)²=√4-2xy∈[0,√8]
-√8≤x-y≤√8
即x-y≥-2√2