x^2+y^2-4x=0(x-2)^2+y^2=4圆心(2,0)、半径r=2圆心到直线y=kx+2的距离：d=|2k-1*0+2|/√(k^2+(-1)^2)=2|k-1|/√(k^2+1)d^2=4(k-1)^2/(k^2+1)弦长=2√(r^2-d^2)=2√(4-4(k-1)^2/(k^2+1))=4√(1-(k^2-2k+1)/(k^2+1))=4...