a2/4x4/7= 2/7                          b 10/21:5/7=2/3                               c10/21:2/3=5/7                                 d1/5x1/3=1/15           

e1/15:1/5=1/3                          g 1/15:1/5= 1/3                                h 1/15:1/3= 105     

k mình nhé bạn

đm ngu.tao bảo đầy đủ phép tính cơ con phương anh

ta có 
1/2 P=1/2(1-1/10-1/15-1/3-1/28-1/6-1/21)
=1/2-(1/6+1/12+1/20+1/30+1/42+1/56)
=1/2-(1/2.3+1/3.4+1/4.5+1/5.6+1/6.7+1/7.8)
=1/2-(1/2-1/8)
=1/8
suy ra P=1/4

ta có 
1/2 P=1/2(1-1/10-1/15-1/3-1/28-1/6-1/21)
=1/2-(1/6+1/12+1/20+1/30+1/42+1/56)
=1/2-(1/2.3+1/3.4+1/4.5+1/5.6+1/6.7+1/7.8)
=1/2-(1/2-1/8)
=1/8
suy ra P=1/4

A=/x-2/+3x+2

M=1/10 + 1/15 + 1/21 +....+  1/120

M=2/20 +2/30+2/42+....+2/240

M=2/4.5 + 2/5.6 + 2/6.7 +.....+ 2/15.16

M=2.(1/4.5 +......+ 1/15.16)

M=2.(1/4 -1/5 +1/5 - 1/6 +.....+ 1/15 - 1/16)

M=2.(1/4 - 1/16)

M=2.(4/16 - 1/16)

M=2. 3/16

M=6/16=3/8 

Có 1/3 = 8/24 < 9/24 = 3/8 =>1/3<M

Có 1/2 = 4/8>3/8 =>1/2 >M

=> 1/3 < M < 1/2

cảm ơn bạn rất nhiều

k cho mik

Ta có:

\(A=\frac{10^{15}+1}{10^{16}+1}=\frac{10^{16}+10}{10^{16}+1}=\frac{10^{16}+1+9}{10^{16}+1}=1+\frac{9}{10^{16}+1}\)

\(B=\frac{10^{16}+1}{10^{17}+1}=\frac{10^{17}+10}{10^{17}+1}=\frac{10^{17}+1+9}{10^{17}+1}=1+\frac{9}{10^{17}+1}\)

Vì \(\frac{1}{10^{16}+1}>\frac{1}{10^{17}+1}\)

\(\Rightarrow\frac{9}{10^{16}+1}>\frac{9}{10^{17}+1}\)

\(\Rightarrow1+\frac{9}{10^{16}+1}>1+\frac{9}{10^{17}+1}\)

\(\Rightarrow A>B\)

bạn tham khảo nha: hoc24.vn/hoi-dap/question/31717.html

Bạn xem lại đề 

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

\(\frac{1}{2}A=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+...+\frac{1}{9.10}\)

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

\(\frac{1}{2}A=1-\frac{1}{10}\)

\(\frac{1}{2}A=\frac{9}{10}\)

\(A=\frac{9}{10}:\frac{1}{2}\)

\(A=\frac{18}{10}=\frac{9}{5}\)

sao thế tự dưng lấy 1/12 ở đâu thế 1/10 cơ mà

a: =18/70+4/5=18/70+56/70=74/70=37/35

b: =4/5-3/4=16/20-15/20=1/20

a) \(\dfrac{2}{7}\times\dfrac{9}{10}+\dfrac{4}{5}=\dfrac{18}{70}+\dfrac{4}{5}=\dfrac{9}{35}+\dfrac{4}{5}=\dfrac{9}{35}+\dfrac{28}{35}=\dfrac{37}{35}\)

b) \(\dfrac{12}{15}-\dfrac{1}{2}:\dfrac{2}{3}=\dfrac{12}{15}-\dfrac{1}{2}\times\dfrac{3}{2}=\dfrac{12}{15}-\dfrac{3}{4}=\dfrac{4}{5}-\dfrac{3}{4}=\dfrac{16}{20}-\dfrac{15}{20}=\dfrac{1}{20}\)