a, \(\frac{5}{11.16}+\frac{5}{11.21}+\frac{5}{21.26}+...+\frac{5}{61.66}\)

=\(\frac{1}{11}-\frac{1}{16}+\frac{1}{16}-\frac{1}{21}+\frac{1}{21}-\frac{1}{26}+...+\frac{1}{61}-\frac{1}{66}\)

=\(\frac{1}{11}-\frac{1}{66}=\frac{5}{66}\)

a, A=1/11-1/16+1/16-1/21+1/21-1/26+...+1/61-1/66

      = 1/11-1/66=5/66 ( A chính là biểu thức ở phần a)

b, 1/12+1/20+1/30+...+1/110

=1/3.4+1/4.5+1/5.6+..+1/10.11

=1/3-1/4+1/4-1/5+1/5-1/6+...+1/10-1/11

=1/3-1/11=8/33

b,\(\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+...+\frac{1}{10.11}\) 

=\(\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+...+\frac{1}{10}-\frac{1}{11}\)

=\(\frac{1}{2}-\frac{1}{11}=\frac{9}{22}\)

T=1/2+5/6+11/12+...+89/90+109/110+10/11
T= (1 -1/2) + ( 1 - 1/6) + (1-1/12) + (1-1/20)+(1-1/30)+... + (1 - 1/90) + (1- 1/110) + (10/11)
T= 1x10 –(1/2+1/6+1/12+1/20+……..+1/90+1/110)+10/11
T=10 – (1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+……+1/9-1/10+1/10-1/11)+ 10/11
T= 10 – (1-1/11)+10/11 = 10-10/11+10/11
T=10
 

1/2 + 5/6 +11/12 +...+ 89/90 + 109/110 + 10/11

= (1 - 1/2) + ( 1 - 1/6) + (1 - 1/12) + (1 - 1/20) +( 1 - 1/30) +... + (1 - 1/90) + (1- 1/110) + 10/11

= 1 x 10 –(1/2 + 1/6 + 1/12 + 1/20 +……..+ 1/90 + 1/110) + 10/11

= 10 – (1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + 1/4 - 1/5 +……+ 1/9 - 1/10 + 1/10 - 1/11 ) + 10/11

= 10 – (1 - 1/11) + 10/11

= 10 - 10/11 + 10/11

= 10

dễ nhưng ko tính

\(=\frac{111}{11}\)

111/11

tick chao minh nha

là 111/11

tik nha bạnNguyễn thị Hằng  yêu quý!