giúp mik ạ

Sửa đề: bỏ 1/30*40

\(A=\dfrac{1}{5}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{12}+\dfrac{1}{12}-\dfrac{1}{19}+...+\dfrac{1}{28}-\dfrac{1}{39}\)

=1/5-1/39

=34/195

Còn B là biểu thức nào vậy bạn?

Sửa đề: 39*40

\(A=\dfrac{1}{5}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{12}+...+\dfrac{1}{39}-\dfrac{1}{40}=\dfrac{1}{5}-\dfrac{1}{40}=\dfrac{7}{40}\)

\(B=\dfrac{2}{3}\left(\dfrac{1}{5\cdot8}+\dfrac{1}{8\cdot11}+...+\dfrac{1}{17\cdot20}\right)\)

\(=\dfrac{2}{3}\left(\dfrac{1}{5}-\dfrac{1}{8}+\dfrac{1}{8}-\dfrac{1}{11}+...+\dfrac{1}{17}-\dfrac{1}{20}\right)\)

=2/3*3/20=2/20=1/10=4/40<A

  4/3x7 + 5/7x12 + 1/12x13 + 7/13x20 + 3/20x23

=1/3-1/7+1/7-1/12+1/12-1/13+1/13-1/20+1/20-1/23

=1/3-1/23

=20/69

Kanzaki Mizuki

\(=\frac{1}{3}-\frac{1}{7}+\frac{1}{7}-\frac{1}{12}+\frac{1}{12}-\frac{1}{13}+\frac{1}{13}-\frac{1}{20}+\frac{1}{20}-\frac{1}{23}\)

\(=\frac{1}{3}-\frac{1}{23}\)

\(=\frac{20}{69}\)

20/69

= 20 / 69 đó 

a = 1/5-1/8 +1/8-1/11+....+1/602-1/605 = 1/5-(1/8-1/8) -(1/11-1/11)-......-(1/602-1/602)-1/605

1/5-1/605=120/605=24/121 

câu b bạn làm tương tự nhé 

\(\frac{16x17-5}{16x16+11}=\frac{16x16+16-5}{16x16+11}=1\)

\(\frac{4}{3x7}+\frac{5}{7x12}+\frac{1}{12x13}+....+\frac{3}{20x23}\)

=\(\frac{1}{3}-\frac{1}{7}+\frac{1}{7}-\frac{1}{12}+\frac{1}{12}-\frac{1}{13}+...+\frac{1}{20}-\frac{1}{23}\)

=\(\frac{1}{3}-\frac{1}{23}=\frac{20}{69}\)

a) \(\frac{16\cdot17-5}{16\cdot16+11}\)

= \(\frac{16\cdot16+16-5}{16\cdot16+11}\)

= \(\frac{16\cdot16+11}{16\cdot16+11}=1\)