O:x^2+y^2=9,O(0,0)
C:x^2+y^2-4x+4y-1=0,C(2,-2)
OC的中点M(1,-1),k(OC)=-1
k(L)=-1/k(OC)=1
y+1=k(L)*(x-1)=x-1
L:x-y-2=0