2+2/3+2/6+...+2/x*(x+1)=1*1989/1991
Những câu hỏi liên quan
2+ 2/3+2/6+2/12+...+2/x(x+1)= 1 1989/ 1991
tìm x biết :
2+2/3+2/6+2/12+....+2/x*(x+1) = 1+1989/1991
1+1\3+1/6+1/10+.............+2/x*(x+1)=1 1989/1991
ta có
\(1+\frac{1}{3}+\frac{1}{6}+..+\frac{2}{x\left(x+1\right)}=1+2\left(\frac{1}{2}-\frac{1}{3}\right)+2\left(\frac{1}{3}-\frac{1}{4}\right)+..+2\left(\frac{1}{x}-\frac{1}{x+1}\right)=2-\frac{2}{x+1}\)
Nên ta có
\(2-\frac{2}{x+1}=1+\frac{1989}{1991}\Leftrightarrow\frac{2}{x+1}=\frac{2}{1991}\Leftrightarrow x=1990\)
2 + 2/3 + 2/6 + 2/12 +...+ 2/x.(x+1) = 1+1989/1991
Giải đầy đủ hộ mình nhé!
2 + 2/3 + 2/6 + 2/12 +...+ 2/x.(x+1) = 1+1989/1991
Giải đầy đủ hộ mình nhé!
tim x
2+2/3+2/6+2/12+...+2/x*(x+1)=1+1989/1991
giúp mình nha mình đang rất cần
=>2+2/3+2/2x3+2/3x4+......+2/x*(x+1)=1989/1991
=>2+2/3+1/2-1/3+1/3-1/4+.......+1/x-1/x+1=1989/1991
=>tự tính nốt
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1+1/3+1/6+1/10+.......+2/Xx(X+1)=1và 1989/1991
ta có
\(1+\frac{1}{3}+\frac{1}{6}+...+\frac{2}{x\left(x+1\right)}\) \(=\)\(1+2\)\(\left(\frac{1}{2}-\frac{1}{3}\right)+2\left(\frac{1}{3}-\frac{1}{4}\right)+...+2\left(\frac{1}{x}-\frac{1}{x+1}\right)\)\(=2-\frac{2}{x+1}\)
Nên ta có
\(2-\frac{2}{x+1}=1+\frac{1989}{1991}\Leftrightarrow\frac{2}{x+1}=\frac{2}{1991}\Leftrightarrow x=1990\)
1 + \(\dfrac{1}{3}\) + \(\dfrac{1}{6}\) + \(\dfrac{1}{10}\)
+......+ \(\dfrac{2}{x(x+1)}\) =1\(\dfrac{1989}{1991}\)
HeLp me
Tìm x
2 + 2/3 + 2/6 +2/12 + ...+ 2/x(x+1)= 1+ 1989 /1991( hỗn số 1 và 1989/1991)