1/56+1/72+1/90+1/110 2/3+2/9+2/27+2/81+2/243+2/729两道题是分开的。打错了。应该是2/3+2/9+2/27+2/81+2/243+2/729

1/56+1/72+1/90+1/110 2/3+2/9+2/27+2/81+2/243+2/729两道题是分开的。打错了。应该是2/3+2/9+2/27+2/81+2/243+2/729
1/56+1/72+1/90+1/110 2/3+2/9+2/27+2/81+2/243+2/729
两道题是分开的。打错了。应该是2/3+2/9+2/27+2/81+2/243+2/729

1/56+1/72+1/90+1/110 2/3+2/9+2/27+2/81+2/243+2/729两道题是分开的。打错了。应该是2/3+2/9+2/27+2/81+2/243+2/729
1题：
1/56+1/72+1/90+1/110
=(1/7-1/8)+(1/8-1/9)+(1/9-1/10)+(1/10-1/11)
=1/7-1/8+1/8-1/9+1/9-1/10+1/10-1/11
=1/7-1/11
=11/77-7/77
=4/77
2题：
2/3+2/9+2/27+2/81+2/243+2/729
=(1-1/3)+(1/3-1/9)+(1/9-1/27)+(1/27-1/81)+(1/81-1/243)+(1/243-1/729)
=1-1/3+1/3-1/9+1/9-1/27+1/27-1/81+1/81-1/243+1/243-1/729
=1-1/729
=728/729

原式=(1+2++10)+(1/2+1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90+1/110) =10(10+1)/2+[(1-1/2)+(1/2-1/3)++(

看不懂

(1)原式=1/(7x8)+1/(8x9)+1/(9x10)+1/(10x11)
=(1/7-1/8)+(1/8-1/9)+(1/9-1/10)+(1/10-1/11)
=1/7-1/11
=4/77
（2）楼上的很强。