❤minh cung dang thac mac cau hoi nay

Ta có: 
7/12 = 4/12 + 3/12 = 1/3 + 1/4 = 20/60 + 20/80 

1/41 + 1/42 + 1/43 +...+ 1/79 + 1/80 = (1/41 + 1/42 + 1/43 + ...+ 1/60) + (1/61 + 1/62 +...+ 1/79 + 1/80) 

Do 1/41> 1/42 > 1/43 > ...>1/59 > 1/60 
=> (1/41 + 1/42 + 1/43 + ...+ 1/60) > 1/60 + ...+ 1/60 = 20/60 

và 1/61> 1/62> ... >1/79> 1/80 
=> (1/61 + 1/62 +...+ 1/79 + 1/80) > 1/80 + ...+ 1/80 = 20/80 

Vậy: 1/41 + 1/42 + 1/43 +...+ 1/79 + 1/80 > 20/60 + 20/80 = 7/12 

=> 1/41 + 1/42 + 1/43 +...+ 1/79 + 1/80 > 7/12 

=> ĐPCM

Ta có: (1/41+/42+1/43+...+1/60)+(1/61+1/62+1/63+...+1/80)>(1/60+1/60+1/60+...+1/60)+(1/80+1/80+1/80+...+1/80)(có 20 số 1 ngoặc)                          
                                                                                     =                 20/60             +         20/80    =1/3 +1/4     =7/12

Vậy phép tính trên lớn hơn 7/12  nhé                                                           
    

S = 1 + 2 - 3 - 4 + 5 + 6 - 7 - 8 + ... + 994 - 995 - 996 + 997 + 998

S = 1 + 2 + 5 − 3 − 4 + 6 + 9 − 7 − 8+. . . +994 + 997 − 995 − 996 + 998

S = 1 + (2 + 5 − 3 − 4) + (6 + 9 − 7 − 8) +. . . + (994 + 997 − 995 − 996) + 998

S = 1 + 0 + 0+. . . +0 + 998

S = 1 + 998

S = 999

S = 1 + 2 - 3 - 4 + 5 + 6 - 7 - 8 + 9 + ... + 994 - 995 - 996 + 997 + 998

S = 1 + ( 2 + 5 - 3 - 4 ) + ( 6 + 9 - 7 - 8 ) + ... + ( 994 + 997 - 995 - 996 ) + 998

S = 1 + 0 + 0 + ... + 0 + 998

S = 1 + 998 

S = 999

#Zin-hồly

mọi người làm đi

mình k cho

bằng 0.045

a, bạn sửa lại đề nhé 

b, \(C=\frac{2n+1}{4n+6}=\frac{4n+4}{4n+6}=\frac{4n+6-2}{4n+6}=1-\frac{2}{4n+6}=1-\frac{1}{2n+3}\)

\(\Rightarrow2n+3\inƯ\left(1\right)=\left\{\pm1\right\}\)

\(D=\frac{2n+1}{n-3}=\frac{2\left(n+\frac{1}{2}\right)}{n-3}=\frac{2\left(n-3+\frac{7}{2}\right)}{n-3}\)

\(=\frac{2\left(n-3\right)+7}{n-3}=2+\frac{7}{n-3}\Rightarrow n-3\inƯ\left(7\right)=\left\{\pm1;\pm7\right\}\)

\(\Leftrightarrow x\left(\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{2020.2021}\right)=-1\)

\(\Leftrightarrow x\left(\frac{2-1}{1.2}+\frac{3-2}{2.3}+\frac{4-3}{3.4}+...+\frac{2021-2020}{2020.2021}\right)=-1\)

\(\Leftrightarrow x\left(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{2020}-\frac{1}{2021}\right)=-1\)

\(\Leftrightarrow x\left(1-\frac{1}{2021}\right)=-1\Leftrightarrow x.\frac{2020}{2021}=-1\Rightarrow x=-\frac{2021}{2020}\)