m²-5m-4
=m^2-5m+(5/2)^2-4-(5/2)^2
=(m-5/2)^2-41/4
=(m-5/2)^2-（√41/2)^2
=(m-5/2+√41/2) (m-5/2-√41/2)
=[m-(5-√41)/2]*[ m-(5/2+√41)/2]