2/5÷（-2 2/5）-8/21×（-1 3/4）-0.25 最好有什么简便计算提示（运算律)
问题描述：
2/5÷（-2 2/5）-8/21×（-1 3/4）-0.25 最好有什么简便计算提示（运算律)
过程
答
2/5 ÷ (-2 2/5) - 8/21 × (-1 3/4) - 0.25
= 2/5 ÷ (-12/5) - 8/21 × (-7/4) - 0.25
= 2/5 × (-5/12) - 8/21 × (-7/4) - 0.25
= -1/6 - (-2/3) - 0.25
= -1/6 + 4/6 - 0.25
= 3/6 - 0.25
= 0.5 - 0.25
= 0.25