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

问题描述:

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

m^2=20+mn n^2=8+mn
带入 =20+mn-40-5mn+4mn
=-20