1. (5m²+4m²n-5mn²) - (5m²+4m²n-3mn²) = 5m²+4m²n-5mn²-5m²-4m²n+3mn² = -2mn²
2. (-y³-3x⁴+x²y) + (4x⁴+9x²y-2y³) = = -y³-3x⁴+x²y + 4x⁴+9x²y-2y³ = -3y³+x⁴+10x²y
3. (-5x²y+2y³-x⁴)+(-5x⁴+5x²y+8y³) = -5x²y+2y³-x⁴-5x⁴+5x²y+8y³ = 10y³-6x⁴
4. (-4cd-c³+d²)+(2c³-d²)=-4cd-c³+d²+2c³-d²=c³-4cd
5. (2x²y-y-3x⁴)+(-7y-3x⁴)=2x²y-y-3x⁴-7y-3x⁴=2x²y-8y.