C1:X^2+Y^2+4y=0,C2:X^2+Y^2+2(a-1)x+2y+a^2=0
圆心C1（0,-2）半径R1^2=4
圆心C2（1-a,-1）半径R2^2=2-2a>0,a<1
切线互相垂直:
C1C2^2=R1^2+R2^2
(1-a)^2+(-2+1)^2=2^2+2-2a
a=±2,但a<1
所以实数a的值
a=-2