应该是m²+n²=5,否则无法做.
(5+2m²+3n²-mn)-(4m²+5n²-mn)
=5+2m²+3n²-mn-4m²-5n²+mn
=(2m²-4m²)+(3n²-5n²)+(-mn+mn)+5
=-2m²-2n²+5
=-2(m²+n²)+5
把m²+n²=5代入上式得
=-2×5+5
=-5
答案:-5
应该是m²+n²=5,否则无法做.
(5+2m²+3n²-mn)-(4m²+5n²-mn)
=5+2m²+3n²-mn-4m²-5n²+mn
=(2m²-4m²)+(3n²-5n²)+(-mn+mn)+5
=-2m²-2n²+5
=-2(m²+n²)+5
把m²+n²=5代入上式得
=-2×5+5
=-5
答案:-5