\(1-2+3-4+5-6+7-8...199-200\)
\(=\left(1-2\right)+\left(3-4\right)+\left(5-6\right)+\left(7-8\right)...+\left(199-200\right)\)
\(=\left(-1\right)+\left(-1\right)+\left(-1\right)+\left(-1\right)...+\left(-1\right)\) \((\)Có 100 số -1)
\(=\left(-1\right).100\)
\(=-100\)