Bài `4`:
`a.`
`+) C(x)=2x^5-4x^4+3x^3-x^2+5x-1-x^5+2x^4-3x^3-x^2-2x+7`
`= x^5-2x^4-2x^2+3x+6`
`+) D(x)=2x^5-4x^4+3x^3-x^2+5x-1+x^5-2x^4+3x^3-x^2+2x-7`
`= 3x^5-6x^4+6x^3+7x-8`
`b.`
`+) C(0)=0^5-2.0^4-2.0^2+3.0+6`
`= 0-0-0+0+6`
`= 6`
`+) D(-1)=3.(-1)^5-6.(-1)^4+6.(-1)^3+7.(-1)-8`
`= -3-6-6-7-8`
`= -30`
Bài `5`:
`a. 4x+8=0`
`<=> x=-8/4`
`<=> x=-2`
`b. 5x-1/3=0`
`<=> 5x=1/3`
`<=> x=1/3 .1/5`
`<=> x=1/15`
`c. x^2+4=0`(vô lí, `x^2\ge0`)
`=> x\in∅`
`d. x^2-2x=0`
`<=> x=0` hoặc `x=2`
