(1/11)-(11/31)+(2/11)-(12/31)+(3/11）-(13/31）+……(10/11)-(20/31) 的简便运算
=【(1/11)+(2/11)+(3/11）+…+(10/11)】-【(11/31) +(12/31)+(13/31）+...+(20/31)】
=(1+2+3+...+10)/11-(11+12+13+...+20)/31
=55/11-155/31
=5-5=0