1）1-2+3-4+……+2005-2006
=(1-2)+(3-4) 一共有1003组,每和都为-1
=1003x(-1)
=-1003
2) 69+699+6999+69999+699999
=70-1+700-1+7000-1+70000-1+700000-1
=777770-5
=777765