两点距离[(x1-x2)^2+(y1-y2)^2]^(1/2)[(a-1)^2+(2-1)^2]^1/2=5(a^2-2a+1+1)^0.5=5a^2-2a+2=25a^2-2a-23=0a=1+2*根号6,1-2*根号6
