C1:
x=2t+2a (1)
y=-t (2)
sub (2) into (1)
x=-2y+2a (3)
C2:
x=2cosθ (4)
y=2+2sinθ
y-2=2sinθ (5)
(4)^2+(5)^2
x^2+(y-2)^2 = 4 (6)
sub (3) into (6)
(-2y+2a)^2+(y-2)^2 = 4
5y^2-(4+8a)y+4a^2=0
判别式
=(4+8a)^2-4(5)(4a^2) >=0
16a^2-64a-16≤0
a^2-4a-1≤0
2-√5≤a≤2+√5