16m-m³
=-m(m²-16)
=-m(m+4)(m-4)
m²+m-20
=m²+5m-4m-20
=m(m+5)-4(m+5)
=(m+5)(m-4)第二个的m²+5m-4m-20...哪儿来的5m和4m?m=5m-4m