1-2=-1 3-4=-1……99-100=-1 (即两个一组,和为-1)
n-2n+3n-4n+5n-6n+...+99n-100n
=50(-1)n
=-50n
=(-50)(-1)
=50
若n等于-1,则n-2n+3n-4n+5n-6n+······+99n-100n的值为（ 50 ）