设圆为(x-a)^2+(y-a)^2=r^2
截y轴所得弦长为2 =>r^2=1+a^2
与x+2y--1=0相切 =>|a+2a-1|/根号5=r
联立解得a=-1/2或2
方程是(x-2)^2+(y-2)^2=5
或(x+1/2)^2+(y+1/2)^2=5/4