$1+2-3-4+5+6-7-8+...-2007-2008+2009$

$=\left(1+2-3-4\right)+\left(5+6-7-8\right)+...+\left(2005+2006-2007-2008\right)+2009$

$=\left(-4\right)+\left(-4\right)+\left(-4\right)+...+\left(-4\right)+2009$

$=\left(-4\right)\cdot 502+2009$

$=\left(-2008\right)+2009$

$=1$

I am thirsty. I 'd like some water

$=\genfrac{}{}{0ex}{}{2006\cdot 2005+2006+2006-1997}{2006\cdot 2005+2015}$

$=\genfrac{}{}{0ex}{}{2006\cdot 2005+4012-1997}{2006\cdot 2005+2015}$

$=\genfrac{}{}{0ex}{}{2006\cdot 2005+2015}{2006\cdot 2005+2015}=1$

$2006\cdot 2007-1997$

$=\genfrac{}{}{0ex}{}{2006\cdot 2005+2006+2006-1997}{2006\cdot 2005+2015}$

$=\genfrac{}{}{0ex}{}{2006\cdot 2005+4012-1997}{2006\cdot 2005+2015}$

$=\genfrac{}{}{0ex}{}{2006\cdot 2005+2015}{2006\cdot 2005+2015}=1\; nha\; bạn.$