x^2+y^2=4,圆心O(0,0)到直线x-y-c=0的距离
d=|0+0-c|/√(1^2+1^2)=|c|/√2
圆x^2+y^2=4半径为2,
(1)当d=2:|c|=2√2,即c=±2√2时相切
(2)d