Lời giải:
`f)`
`5x(x-3)^(2)-5(x-1)^(3)+15(x+2)(x-2)=5`
`⇔5x(x^(2)-6x+9)-5(x^(3)-3x^(2)+3x-1)+15(x^(2)-4)=5`
`⇔5x^(3)-30x^(2)+45x-5x^(3)+15x^(2)-15x+5+15x^(2)-60=5`
`⇔(5x^(3)-5x^2)+(-30x^(2)+15x^(2)+15x^2)+(45x-15x)+(5-60)=5`
`⇔30x-55=5`
`⇔30x=60`
`⇔x=2`
Vậy `x=2`
`g)`
`(x+3)^(3)-x(3x+1)^(2)+(2x+1)(4x^(2)-2x+1)-3x^2=42`
`⇔x^(3)+9x^(2)+27x+27-x(9x^(2)+6x+1)+(2x)^(3)+1^(3)-3x^2=42`
`⇔x^(3)+9x^(2)+27x+27-9x^(3)-6x^(2)-x+8x^(3)+1-3x^2=42`
`⇔(x^(3)-9x^3+8x^3)+(9x^(2)-6x^(2)-3x^2)+(27x-x)+(27+1)=42`
`⇔26x+28=42`
`⇔26x=14`
`⇔x=7/13`
Vậy `x=7/13`
