Tim so nguyen x , biet : 1 phan 6 = x phan 18 ; x phan 8 = -1 phan 4 ; 4 phan -5 = x phan 10 ; 11 phan 5 = -22 phan x ; x phan 8 = 8 phan x ; x phan -11 = -11 phan x
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\(\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+....+\frac{1}{x\left(x+1\right)}=\frac{2008}{2009}\)
\(=>\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+....+\frac{1}{x\left(x+1\right)}=\frac{2008}{2009}\)
\(=>\frac{1}{1}-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{x}-\frac{1}{x+1}=\frac{2008}{2009}\)
\(=>1-\frac{1}{x+1}=\frac{2008}{2009}=>\frac{1}{x+1}=1-\frac{2008}{2009}=\frac{1}{2009}\)
=>x+1=2009
=>x=2008
Vậy x=2008
1/2+1/6+1/12+...+1/x*(x+1)=2008/2009
1/1*2+1/2*3+1/3*4+...+1/x*(x+1)=2008/2009
1-1/2+1/2-1/3+1/3-1/4+...+1/x-1/(x+1)=2008/2009
1-1/x+1)=2008/2009
1/x+1=1-2008/2009
1/x+1=1/2009
nên x+1=2009
x=2009-1
x=2008 (tick nha)
Neu dem tu cua phan so tru di A va them A vao mau thi tong khong thay doi
Tong cua mau so va tu so la : 8 + 7 = 15
Coi mau so moi la 4 phan bang nhau thi tu so moi la mot phan nhu the
Tu so moi la : 15 / ( 1 +4 ) x 1 = 3
So A la : 7 - 3 = 4
DS : 4
\(\frac{1}{6}=\frac{x}{18}\Rightarrow x=3\)
\(\frac{x}{8}=-\frac{1}{4}\Rightarrow x=-2\)
\(\frac{4}{-5}=\frac{x}{10}\Rightarrow x=-8\)
\(\frac{11}{5}=-\frac{22}{x}\Rightarrow x=-10\)
\(\frac{x}{8}=\frac{8}{x}\Rightarrow x^2=64\Rightarrow x=8\)
\(\frac{x}{-11}=-\frac{11}{x}\Rightarrow x^2=121\Rightarrow x=11\)
#H