Giải:
a) \(\left(-1\right).\left(-2\right).\left(-3\right).\left(-4\right).\left(-5\right).\left[\left(-3\right)-\left(-5\right)\right]\)
\(=2.12.\left(-5\right).2\)
\(=24.\left(-10\right)\)
\(=-240\)
b) \(1-2+3-4+5-6+...-98+99\)
\(=\left(1-2\right)+\left(3-4\right)+\left(5-6\right)+...+\left(97-98\right)+99\)
\(=\left(-1\right)+\left(-1\right)+\left(-1\right)+...+\left(-1\right)+99\)
\(=49.\left(-1\right)+99\)
\(=-49+99\)
\(=50\)
Chúc bạn học tốt!
\(a)\left(-1\right)\left(-2\right)\left(-3\right)\left(-4\right)\left(-5\right).\left[\left(-3\right)-\left(-5\right)\right]\)
\(=\left(-120\right).2\)
\(=\left(-240\right)\)
\(b)1-2+3-4+5-6+...-98+99\)
\(=\left(-1\right)+\left(-1\right)+\left(-1\right)+...+\left(-1\right)\)( có 50 cặp số - 1 )
\(=\left(-1\right).50\)
\(=\left(-50\right)\)