M+N+K:
2x⁴+x³y-3x²y²+4xy³-y⁴-3x⁴+2x³y+5x²y²+y⁴+x⁴-x³y-2x²y²+4xy³-2y⁴=-2y⁴+8xy³+2x³y=2y(-y³+4xy²+x³)
M-N+K:
2x⁴+x³y-3x²y²+4xy³-y⁴-(-3x⁴+2x³y+5x²y²+y⁴)+x⁴-x³y-2x²y²+4xy³-2y⁴=2x⁴+x³y-3x²y²+4xy³-y⁴+3x⁴-2x³y-5x²y²-y⁴+x⁴-x³y-2x²y²+4xy³-2y⁴=4x⁴-2x³y-10x²y²+8xy³-4y⁴
M-N-K:
2x⁴+x³y-3x²y²+4xy³-y⁴-(-3x⁴+2x³y+5x²y+y⁴)-(x⁴-x³y-2x²y²+4xy³-2y⁴)=2x⁴+x³y-3x²y²+4xy³-y⁴+3x⁴-2x³y-5x²y²-y⁴-x⁴+x³y+2x²y²-4xy³+2y⁴=4x⁴-2x³y-6x²y²
- M+N+K:
-(2x⁴+x³y-3x²y²+4xy³-y⁴)-3x⁴+2x³y+5x²y²+y⁴+x⁴-x³y-2x²y²+4xy³-2y⁴=-2x⁴-x³y+3x²y²-4xy³+y⁴-3x⁴+2x³y+5x²y²+y⁴+x⁴-x³y-2x²y²+4xy³-2y⁴= -4x⁴+6x²y²
