a ) 3/5 + 3/16 + 13/16 

= 3/5 + ( 3/16 + 13/16 ) 

= 3/5 + 16/16

= 3/5 + 1

= 3/5 + 5/5

= 8/5 

b ) 1/1 x 2 + 1/ 2 x 3 + 1/ 3 x 4

= 1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4

= 1 - 1/4

= 4/4 - 1/4

= 3/4 

a) 3/5+3/16+13/16

=3/5+16/16

=3/5+1

=3/5+5/5

=8/5

a      =3/5+(3/16+13/16)=3/5+16/16=3/5+1=8/5

b        =1-1/2+1/2-1/3+1/3-1/4=1-1/4=3/4

(ở đầu câu mình để dấu bằng để trước dấu bằng bạn viết lại đề bài na)

\(\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+....+\frac{1}{15.16}\)

\(=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+.....+\frac{1}{15}-\frac{1}{16}\)

\(=1-\frac{1}{16}\)

\(=\frac{15}{16}\)

Kết quả là 15/16 nha bn, k mk nhé, ok

15/16 k mình nha các bn

a) 3/7 + 4/9 + 4/7 + 5/9

= ( 3/7 + 4/7 ) + ( 4/9 + 5/9 )

= 7/7 + 9/9

= 1  + 1 

= 2

b)1/5 + 4/10 + 9/15 + 16/20 + 25/25 + 36/30 + 49/35 + 64/40 + 81/45

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

= ( 1/5 + 9/5 ) + ( 2/5 + 8/5 ) + (7/5 + 3/5 ) + ( 4/5 + 6/5  ) + 5/5

= 2 + 2 + 2 + 2 + 1

= 2  x 4 + 1

= 8  +1 

= 9

c)  1/8 + 1/12 + 3/8 + 5/12

= ( 1/8 + 3/8 ) + ( 1/12 + 5/12)

= 4/8 + 6/12

= 1/2 + 1/2

= 2/4 = 1/2

mỏi tay rồi

\(\dfrac{1}{1\times2}+\dfrac{1}{2\times3}+\dfrac{1}{3\times4}+...+\dfrac{1}{\left(x-1\right)\times x}=\dfrac{15}{16}\)

\(1-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+...+\dfrac{1}{x-1}-\dfrac{1}{x}=\dfrac{15}{16}\)

\(1-\dfrac{1}{x}=\dfrac{15}{16}\)

\(\dfrac{1}{x}=1-\dfrac{15}{16}=\dfrac{16}{16}-\dfrac{15}{16}\)

\(\dfrac{1}{x}=\dfrac{1}{16}\)

\(\Rightarrow x=16\)

?

TÍNH BẰNG CÁCH NHANH NHẤT NHA CÁC BN 

a) \(\left(\frac{1}{3}+\frac{1}{5}\right)+\left(\frac{1}{6}-\frac{1}{5}\right)=\left(\frac{1}{3}+\frac{1}{6}\right)+\left(\frac{1}{5}-\frac{1}{5}\right)=\frac{1}{2}\)

b) \(\frac{3}{16}\times\frac{7}{5}+\frac{3}{5}\times\frac{9}{16}=\frac{21}{80}+\frac{27}{80}=\frac{48}{80}=\frac{3}{5}\)

c) \(\frac{1}{1\times2}+\frac{1}{2\times3}+...+\frac{1}{2020\times2021}=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+...+\frac{1}{2020}-\frac{1}{2021}\)

\(=1-\frac{1}{2021}=\frac{2020}{2021}\)

d) \(\frac{1}{1\times3}+\frac{1}{3\times5}+...+\frac{1}{2021\times2023}=\frac{1}{2}\times\left(\frac{2}{1\times3}+\frac{2}{3\times5}+...+\frac{2}{2021\times2023}\right)\)

\(=\frac{1}{2}\times\left(1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+...+\frac{1}{2021}-\frac{1}{2023}\right)\)

\(=\frac{1}{2}\times\left(1-\frac{1}{2023}\right)=\frac{1}{2}\times\frac{2022}{2023}=\frac{1011}{2023}\)

e) \(\frac{3}{2}\times\frac{1}{7}\times\frac{5}{4}+\frac{15}{2}\times\frac{6}{7}\times\frac{1}{4}==\frac{15}{56}+\frac{80}{56}=\frac{95}{56}\)

A=1-1/2+1/2-1/3+1/3-1/4+...+1/64-1/128

A=1-1/128

A=127/128

Vậy A=\(\frac{127}{128}\)

B=1-1/2+1/2-1/3+1/3-1/4+...+1/99-1/100

B=1-1/100

B=99/100

Vậy B=\(\frac{99}{100}\)

khó quá sai thì thôi nha

= 1/1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + .... + 1/49 - 1/50

= 1/1 - 1/50

= 49/50

1/1.2 + 1/2.3 + 1/3.4 +.....+ 1/49.50

= 1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 +......+ 1/49 - 1/50

= 1 - 1/50

= 49/50

1/1.2 + 1/2.3 + 1/3.4 +.....+ 1/49.50 = 1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 +......+ 1/49 - 1/50 = 1 - 1/50 = 49/50