题目

爆急
1/3 +1/9 +1/27+ …+1/243 + 1/729
1+1/3 -7/12 + 9/20-11/30+13/42 -15/56+17/72
1993x19951995- 1995x19931993
注意巧算,答对给你10分以上

提问时间：2020-11-21

答案

1/3 +1/9 +1/27+ …+1/243 + 1/729
=(1/3 +1/9 +1/27+ …+1/243 + 1/729 )×2÷2
=(2/3 +2/9 +2/27+ …+2/243 +2/729)÷2
=(2/3 +2/9 +2/27+ …+2/243 +2/729+1/729-1/729)÷2
=(2/3 +2/9 +2/27+ …+2/243 +1/243-1/729)÷2
=(1-1/729)÷2
=728/729×1/2
=364/729
1+1/3 -7/12 + 9/20-11/30+13/42 -15/56+17/72
=1+1/3-(3+4)/(3×4)+ (4+5)/(4×5)- (5+6)/(5×6)+ (6+7)/(6×7) -(7+8)/(7×8)+ (8+9)/(8×9)
=1+1/3-1/3-1/4+1/4+1/5-1/5-1/6+1/6+1/7-1/7-1/8+1/8+1/9
=1+1/9
=10/9
1993x19951995- 1995x19931993
=1993×1995×10001-1995×1993×10001
=0