$\color{red}{\text{Lyy}}$

Đặt `1991 = x`.

`=> 1991. 1996. 1989 - 1993. 1998. 1995`

`= x(x + 5)(x - 2) - (x + 2)(x - 3)(x + 4)`

`=` ($x^2$ `+ 5x)(x - 2) -` ($x^2$ `- 3x + 2x - 6)(x + 4)`

`=` ($x^2$ `+ 5x)(x - 2) -` ($x^2$ `- x - 6)(x + 4)`

`=` $x^3$ `-` $2x^2$ `+` $5x^2$ `- 10x -` ($x^3$ `+ $4x^2$ -` $x^2$ `- 4x - 6x - 24`)

`=` $x^3$ `+` $3x^2$ `- 10x -` $x^3$ `-` $4x^2$ `+` $x^2$ `+ 4x + 6x + 24`

`=` ($x^3$ `-` $x^3$) `+` ($3x^2$ `+` $x^2$ `-` $4x^2$) `+ (6x + 4x - 10) + 24`

`= 24`.

Đặt `2005 = y`.

`=> 2005. 2010. 2003 - 2007. 2002. 2009`

`= y(y + 5)(y - 2) - (y + 2)(y - 3)(y + 4)`

`=` ($y^2$ `+ 5y)(y - 2) -` ($y^2$ `- 3y + 2y - 6)(y + 4)`

`=` ($y^2$ `+ 5y)(y - 2) -` ($y^2$ `- y - 6)(y + 4)`

`=` $y^3$ `-` $2y^2$ `+` $5y^2$ `- 10y -` ($y^3$ `+` $4y^2$ `-` $y^2$ `- 4y - 6y - 24`)

`=` $y^3$ `-` $2y^2$ `+` $5y^2$ `- 10y -` $y^3$ `-` $4y^2$ `+` $y^2$ `+ 4y + 6y + 24`

`=` ($y^3$ `-` $y^3$) `+` ($-2y^2$ `+` $5y^2$ `-` $4y^2$ `+` $y^2$) `+ (4y + 6y - 10y) + 24`

`= 24.`

 Vậy `1991. 1996. 1989 - 1993. 1998. 1995 = 2005. 2010. 2003 - 2007. 2002. 2009`