\(1-2+3-4+5-6+...+2011-2012\)
\(=\left(1-2\right)+\left(3-4\right)+\left(5-6\right)+...+\left(2011-2012\right)\) => có 1006 cặp
\(=\left(-1\right)+\left(-1\right)+\left(-1\right)+...+\left(-1\right)\)
\(\Leftrightarrow\left(-1\right).1006\)
\(=-1006\)