x²+y²≤4
M{(x,y)|-2<=x<=2,-2<=y<=2}
(x-a)²+y²≤9
N{(x,y)|-3<=x-a<=3,-3<=y<=3}
N{(x,y)|-3+a<=x<=3+a,-3<=y<=3}
-3+a<=-2,a<=1;
3+a>=2,a>=-1
综合得：-1<= a<=1