\(1-2+3-4+5-6+7-8+......+97-98+99\)
\(=\left(1-2\right)+\left(3-4\right)+\left(5-6\right)+\left(7-8\right)+......+\left(97-98\right)+99\)
\(=\left(-1\right)+\left(-1\right)+\left(-1\right)+\left(-1\right)+......+\left(-1\right)+99\) ( có 49 chữ số \(\left(-1\right)\) )
\(=\left(-1\right).49+99\)
\(=-49+99\)
\(=50\)