Question

已知：m^2-mn=20,mn-n^2=-8,求m^2-5n^2+4mn的值

Answer 1

两式相加得m^2-n^2=12,

因为由m^2-mn=20得mn=m^2-20,

所以m^2-5n^2+4mn

=m^2-5n^2+4m^2-80

=5(m^2-n^2)-80

=5×12-80=-20