令x+y=ay=a-x代入x²+a²-2ax+x²=42x²-2ax+(a²-4)=0x是实数，所以△>=04a²-8a²+32>=0a²<=8-2√2<=a<=2√2-2√2<=x+y<=2√2
设x+y=ky=-x+k圆心到直线的距离=半径=2所以|k|/√2=2k=±2√2
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