1/3-1/12

=1/4

Chúc bạn bạn may mắn!

1/12 + 1/20 + 1/30 + 1/42 + 1/56 + 1/72 + 1/90 + 1/110 + 1/132 = 2/3,

 Đúng 100%!

Ai tk cho mình mình tk lại.

\(\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+...+\frac{1}{90}+\frac{1}{110}\)

\(=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+...+\frac{1}{9.10}+\frac{1}{10.11}\)

\(=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+...+\frac{1}{9}-\frac{1}{10}+\frac{1}{10}-\frac{1}{11}\)

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

\(=\frac{10}{11}\)

10/11 nha

chúc bạn học tốt tk nha ^.^

chúc bạn học tốt tk nha ^.^

   \(\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+......+\frac{1}{90}+\frac{1}{110}\)

\(=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+.....+\frac{1}{9}-\frac{1}{10}+\frac{1}{10}-\frac{1}{11}\)

 \(=1-\left(\frac{1}{2}-\frac{1}{2}-\frac{1}{3}-\frac{1}{3}-....-\frac{1}{10}-\frac{1}{10}\right)\frac{1}{11}\)

 \(=1-\frac{1}{11}\)

 \(=\frac{10}{11}\)

\(\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+\frac{1}{72}+\frac{1}{90}\)

\(=\frac{1}{1\times2}+\frac{1}{2\times3}+\frac{1}{3\times4}+\frac{1}{4\times5}+\frac{1}{5\times6}+\frac{1}{6\times7}+\frac{1}{7\times8}+\frac{1}{8\times9}+\frac{1}{9\times10}\)

\(=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}\)

                                                                                                                                                                  \(+\frac{1}{8}-\frac{1}{9}+\frac{1}{9}-\frac{1}{10}\)

( Gạch bỏ các phân số giống nhau )

\(=1-\frac{1}{10}\)

\(=\frac{9}{10}\)

Chúc bn học tốt !!!!

1/2+1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90

=1/1x2+1/2x3+1/3x4+1/4x5+1/5x6+1/6x7+1/7x8+1/8x9+1/9x10

=1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9+1/9-1/10

=1-1/10

=9/10

k đúng cho mk nh!!!

\(S1=\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+\frac{1}{72}+\frac{1}{90}\)

\(S1=\frac{1}{1\cdot2}+\frac{1}{2\cdot3}+\frac{1}{3\cdot4}+\frac{1}{4\cdot5}+\frac{1}{5\cdot6}+\frac{1}{6\cdot7}+\frac{1}{7\cdot8}+\frac{1}{8\cdot9}+\frac{1}{9\cdot10}\)

\(S1=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-\frac{1}{9}+\frac{1}{9}-\frac{1}{10}\)

\(S1=1-\frac{1}{10}\)

\(S1=\frac{9}{10}\)

CHÚC BN HC GIỎI !!!!!!!!!! TỨ DIỆP THẢO

S=\(\frac{1}{1.2}+\frac{1}{2.3}+...............+\frac{1}{9.10}\)

=\(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+...............+\frac{1}{9}-\frac{1}{10}\)

=\(1-\frac{1}{10}\)

=\(\frac{9}{10}\)

\(A=1-1-\dfrac{5}{6}+1+\dfrac{7}{12}-1-\dfrac{9}{20}+1+\dfrac{11}{30}-1-\dfrac{13}{42}+1+\dfrac{15}{56}-1-\dfrac{17}{72}+1+\dfrac{19}{90}\)

\(=1-\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}+\dfrac{1}{4}-\dfrac{1}{4}-\dfrac{1}{5}+...+\dfrac{1}{9}+\dfrac{1}{10}\)

=1/2+1/10

=5/10+1/10=6/10=3/5

Câu 1:

1/2 + 1/6 + 1/12+...+1/90

=1/1×2 + 1/2×3 + 1/3×4 +...+1/9×10

=1/1-1/2+1/2-1/3+1/3-1/4+...+1/9-1/10

(Chắc chỗ này bạn hiểu)

TRIỆT TIÊU CÁC PHÂN SỐ GIỐNG NHAU, TA CÒN:

=1/1 -1/10

=9/10

Câu 2:

2abc = 9 x abc

2000+ abc = 9 x abc

2000            =9 x abc - abc

2000            = 8 × abc

abc               = 2000÷8

abc               = 250

1/2+1/6+1/12+...+1/90=1/(1.2)+1/(2.3)+1/(3.4)+...+1/(9.10)

                                =1/1-1/2+1/2-1/3+1/3-1/4+...+1/9-1/10

                                =1-1/10

                                =9/10

9/10 nha bạn

\(\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+..+\frac{1}{90}=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{9.10}\)

\(=\frac{1}{1}-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{9}-\frac{1}{10}=1-\frac{1}{10}=\frac{9}{10}\)

dấu "." là nhân nhé

\(A=\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+...+\frac{1}{90}\)

\(A=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+...+\frac{1}{9.10}\)

\(A=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+...+\frac{1}{9}-\frac{1}{10}\)

\(A=1-\frac{1}{10}\)

\(A=\frac{9}{10}\)

1/2+1/6+1/12+1/20+...+1/90

=1/1.2+1/2.3+1/3.4+...+1/9.10

=1-1/2+1/2-1/3+1/3-1/4+...+1/9-1/10

=1-1/10

=9/10

\(\Rightarrow\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}...+\frac{1}{9.10}\)

=\(\frac{9}{10}\)