y=2x+bk=2(x1-x2)^2=(x1+x2)^2-4x1x2|AB|=√[(1+k^2)(x1-x2)^2]=√{(1+2^2)[(x1+x2)^2-4x1x2]}=√{5[(x1+x2)^2-4x1x2]}
