2.已知m²-mn+2,mn-n²=5则3m²+2mn-5n²=( )
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2.已知m²-mn+2,mn-n²=5则3m²+2mn-5n²=( )
2.已知m²-mn+2,mn-n²=5则3m²+2mn-5n²=( )
2.已知m²-mn+2,mn-n²=5则3m²+2mn-5n²=( )
m²-mn=2--------1)
mn-n²=5--------2)
1)×3+2)×5,得
3(m²-mn)+5(mn-n²)
=3m²+2mn-5n²
=3×2+5×5
=31