a) Đáp án A.

b) Đáp án C.

Bạn tự ghi lại đề nha !

A = \(\frac{1}{3}\)+ \(\left(\frac{1}{42}+\frac{1}{56}+...+\frac{1}{600}\right)\)

A = \(\frac{1}{3}\)+ \(\left(\frac{1}{6.7}+\frac{1}{7.8}+...+\frac{1}{24.25}\right)\)

A = \(\frac{1}{3}\)+ \(\left(\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+...+\frac{1}{24}-\frac{1}{25}\right)\)

A = \(\frac{1}{3}\)+ \(\left(\frac{1}{6}-\frac{1}{25}\right)\)

A = \(\frac{1}{3}\)+    \(\frac{19}{150}\)

A =  \(\frac{23}{50}\).

Em xem lại đề bài nhé! có thể tham khảo bài dưới đây:

\(A=\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+...+\frac{1}{600}\)

\(=\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}+...+\frac{1}{24.25}\)

\(=\frac{6-5}{5.6}+\frac{7-6}{6.7}+\frac{8-7}{7.8}+...+\frac{25-24}{25.24}\)

\(=\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+...+\frac{1}{24}-\frac{1}{25}\)

\(=\frac{1}{5}-\frac{1}{25}=\frac{4}{25}\)

Nếu đề bài không bị sai

\(A=\frac{1}{3}+\left(\frac{1}{42}+...+\frac{1}{600}\right)\)

\(=\frac{1}{3}+\left(\frac{1}{6.7}+\frac{1}{7.8}+...+\frac{1}{24.25}\right)\)

\(=\frac{1}{3}+\left(\frac{1}{6}-\frac{1}{25}\right)\)

Tự làm nốt nhé!

=3/8

0

0

00

000

-HOK TỐT-

\(\frac{27}{20}-\frac{33}{30}+\frac{39}{42}-\frac{45}{56}\)

\(=(\frac{27}{20}-\frac{11}{10})+(\frac{13}{14}-\frac{45}{56})\)

\(=\frac{1}{4}+\frac{1}{8}\)

\(=\frac{3}{8}\)

\(\frac{27}{20}\)-\(\frac{33}{30}\)+\(\frac{39}{42}\)-\(\frac{45}{56}\)

=\(\frac{1}{4}+\frac{1}{8}\)

=\(\frac{3}{8}\)

a) 4 x 8 x ( 56 + 44) - 3200

= 32 x 100 - 3200

= 0

b) \(\frac{3+2}{60}\)+ \(\frac{4+3}{168}\)

= \(\frac{5}{60}\)+ \(\frac{7}{168}\)

= \(\frac{1}{8}\)

A=1/2x3 + 1/3x4 + 1/4x5 + ... + 1/9x10

A=1/2 - 1/3 + 1/3 - 1/4+ 1/4 - 1/5+...+ 1/9 - 1/10

A= 1/2 - 1/10

Vậy A=2/5

Chúc bạn học tốt~~

\(\frac16+\frac{1}{12}+\frac{1}{20}+\cdots+\frac{1}{72}+\frac{1}{90}\)

\(=\frac{1}{2\times3}+\frac{1}{3\times4}+\frac{1}{4\times5}+\ldots+\frac{1}{8\times9}+\frac{1}{9\times10}\)

\(=\frac12-\frac13+\frac13-\frac14+\cdots+\frac18-\frac19+\frac19-\frac{1}{10}\)

\(=\frac12-\frac{1}{10}=\frac{4}{10}=\frac25\)

\(a=\frac12+\frac56+\frac{11}{12}+\frac{19}{20}+\frac{29}{30}+\frac{41}{42}+\frac{55}{56}\)

\(=1-\frac12+1-\frac16+1-\frac{1}{12}+1-\frac{1}{20}+1-\frac{1}{30}+1-\frac{1}{42}+1-\frac{1}{56}\)

\(=7-\left(\frac12+\frac16+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{1}{56}\right)\)

\(=7-\left(1-\frac12+\frac12-\frac13+\frac13-\frac14+\frac14-\frac15+\frac15-\frac16+\frac16-\frac17+\frac17-\frac18\right)\)

\(=7-\left(1-\frac18\right)=7-\frac78=\frac{56}{8}-\frac78=\frac{49}{8}\)

Đề bài:
a = 1/2 + 5/6 + 11/12 + 19/20 + 29/30 + 41/42 + 55/56

Giải:
Ta thấy mỗi phân số có tử số = mẫu số trừ 1
Vậy ta viết lại:

a = (2 - 1)/2 + (6 - 1)/6 + (12 - 1)/12 + (20 - 1)/20 + (30 - 1)/30 + (42 - 1)/42 + (56 - 1)/56
= 1 - 1/2 + 1 - 1/6 + 1 - 1/12 + 1 - 1/20 + 1 - 1/30 + 1 - 1/42 + 1 - 1/56
= 7 - (1/2 + 1/6 + 1/12 + 1/20 + 1/30 + 1/42 + 1/56)
= 7 - 3/2
= 11/2

Đáp số: 11/2

=\(\frac{619}{120}\)

k cho mk nha