Question

A=1-2+3-4+...+79-80

Answer 1

\(A=1-2+3-4+...+79-80.\)( 80 số ) 

    \(=\left(1-2\right)+\left(3-4\right)+...+\left(79-80\right)\)(40 cặp )

    \(=\left(-1\right)+\left(-1\right)+...+\left(-1\right)\)( 40 số )

    \(=\left(-1\right).40\)

    \(=-40\)

Answer 2

\(A=1-2+3-4+....+79-80.\)

   \(=\left(1-2\right)+\left(3-4\right)+...+\left(79-80\right)\)

   \(=\left(-1\right)+\left(-1\right)+...+\left(-1\right)\)

   \(=\left(-1\right).40\)

   \(=-40\)