爆急1/3+1/9+1/27+…+1/243+1/7291+1/3-7/12+9/20-11/30+13/42-15/56+17/721993x19951995-1995x19931993注意巧算,答对给你10分以上
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
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