`***` Lời giải chi tiết `***`
`a)`
`M(x)=x^{2}+5x^{4}-3x^{3}+x^{2}+4x^{4}+3x^{3}-x+5`
`=(5x^{4}+4x^{4})+(3x^{3}-3x^{3})+(x^{2}+x^{2})-x+5`
`=9x^{4}+2x^{2}-x+5`
``
`N(x)=x-5x^{3}-2x^{2}-8x^{4}+4x^{3}-x+5`
`=-8x^{4}+(4x^{3}-5x^{3})-2x^{2}+(x-x)+5`
`=-8x^{4}-x^{3}-2x^{2}+5`
`b)`
`M(2)=9.2^{4}+2.2^{2}-2+5`
`=9.16+2.4-2+5`
`=144+8-2+5`
`=155`
```
`N(-1)=-8.(-1)^{4}-(-1)^{3}-2.(-1)^{2}+5`
`=-8.1+1-2.1+5`
`=-8+1-2+5`
`=-4`
`c)`
`M(x)+N(x)=9x^{4}+2x^{2}-x+5-8x^{4}-x^{3}-2x^{2}+5`
`=(9x^{4}-8x^{4})-x^{3}+(2x^{2}-2x^{2})-x+(5+5)`
`=x^{4}-x^{3}-x+10`
``
`M(x)-N(x)=9x^{4}+2x^{2}-x+5-(-8x^{4}-x^{3}-2x^{2}+5)`
`=9x^{4}+2x^{2}-x+5+8x^{4}+x^{3}+2x^{2}-5`
`=(9x^{4}+8x^{4})+x^{3}+(2x^{2}+2x^{2})-x+(5-5)`
`=17x^{4}+x^{3}+4x^{2}-x`
