31. 圆C1:x2+y2−4x+1=0C_{1}:x^{2}+y^{2}-4x+1=0C1:x2+y2−4x+1=0与圆C2:x2+y2−2x−2y+1=0C_{2}:x^{2}+y^{2}-2x-2y+1=0C2:x2+y2−2x−2y+1=0的公共弦所在的直线的方程为 ___x−y=0x-y=0x−y=0___,弦长为 [ ].