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Sai đề nhé: Sửa đề:
\(A=\dfrac{5^{100}+2}{5^{102}+2};B=\dfrac{5^{101}+2}{5^{103}+2}\)
nếu:
\(\dfrac{a}{b}< 1\Rightarrow\dfrac{a+m}{b+m}< 1\left(m\in N\right)\)
\(B=\dfrac{5^{101}+2}{5^{103}+2}< 1\)
\(B< \dfrac{5^{101}+2+8}{5^{103}+2+8}\Rightarrow B< \dfrac{5^{101}+10}{5^{103}+10}\Rightarrow B< \dfrac{5\left(5^{100}+2\right)}{5\left(5^{101}+2\right)}\Rightarrow B< \dfrac{5^{100}+2}{5^{101}+2}=A\)
\(B< A\)
nè bạn ơi bạn nhập nó vânx như cũ mà có khác các gì đâu bạn!
Ta có: \(A=\dfrac{1}{1.2}+\dfrac{1}{3.4}+\dfrac{1}{5.6}+...+\dfrac{1}{99.100}\)
\(\Rightarrow A>\dfrac{1}{1.2}+\dfrac{1}{3.4}=\dfrac{1}{2}+\dfrac{1}{12}=\dfrac{14}{24}=\dfrac{7}{12}\)\(\left(1\right)\)
Lại có: \(A=\dfrac{1}{1.2}+\dfrac{1}{3.4}+\dfrac{1}{5.6}+...+\dfrac{1}{99.100}\)
\(A=1-\dfrac{1}{2}+\dfrac{1}{3}-\dfrac{1}{4}+\dfrac{1}{5}+\dfrac{1}{6}+...+\dfrac{1}{99}-\dfrac{1}{100}\)
\(A=\left(1-\dfrac{1}{2}+\dfrac{1}{3}\right)-\left(\dfrac{1}{4}-\dfrac{1}{5}\right)-\left(\dfrac{1}{6}-\dfrac{1}{7}\right)-...-\left(\dfrac{1}{98}-\dfrac{1}{99}\right)-\dfrac{1}{100}\)
\(\Rightarrow A< 1-\dfrac{1}{2}+\dfrac{1}{3}=\dfrac{5}{6}\)\(\left(2\right)\)
Từ \(\left(1\right)\) và \(\left(2\right)\) \(\Rightarrow\dfrac{7}{12}< A< \dfrac{5}{6}\)
Vậy \(\dfrac{7}{12}< A< \dfrac{5}{6}\) ( Điều phải chứng minh ).
Ta có:
\(A=\dfrac{1}{1.2}+\dfrac{1}{3.4}+\dfrac{1}{5.6}+...+\dfrac{1}{99.100}\\ A=\left(\dfrac{1}{1.2}+\dfrac{1}{3.4}\right)+\dfrac{1}{5.6}+...+\dfrac{1}{99.100}\\ A=\dfrac{7}{12}+\dfrac{1}{5.6}+...+\dfrac{1}{99.100}>\dfrac{7}{12}\left(1\right)\\ \Rightarrow A>\dfrac{7}{12}\left(1\right)\)
Ta lại có:
\(A=\dfrac{1}{1.2}+\dfrac{1}{3.4}+\dfrac{1}{5.6}+...+\dfrac{1}{99.100}\\ A=1-\dfrac{1}{2}+\dfrac{1}{3}-\dfrac{1}{4}+\dfrac{1}{5}-\dfrac{1}{6}+...+\dfrac{1}{99}-\dfrac{1}{100}\\ A=\left(1-\dfrac{1}{2}+\dfrac{1}{3}\right)-\left(\dfrac{1}{4}-\dfrac{1}{5}\right)-...\left(\dfrac{1}{98}-\dfrac{1}{99}\right)-\dfrac{1}{100}\\ A=\dfrac{5}{6}-\left(\dfrac{1}{4}-\dfrac{1}{5}\right)-...\left(\dfrac{1}{98}-\dfrac{1}{99}\right)-\dfrac{1}{100}< \dfrac{5}{6}\\ \Rightarrow A=< \dfrac{5}{6}\left(2\right)\)
Từ (1) và (2) suy ra: \(\dfrac{7}{12}< A< \dfrac{5}{6}\left(dpcm\right)\)
a, \(\dfrac{x}{y}=\dfrac{4}{9}\Rightarrow\dfrac{x}{4}=\dfrac{y}{9}\)
Theo tính chất dãy tỉ số bằng nhau ta có :
\(\dfrac{x}{4}=\dfrac{y}{9}=\dfrac{x+y}{4+9}=\dfrac{-30}{13}\)
\(\Rightarrow\left\{{}\begin{matrix}x=4.\left(-\dfrac{30}{13}\right)=\dfrac{-120}{13}\\y=9.\left(-\dfrac{30}{13}\right)=\dfrac{-270}{13}\end{matrix}\right.\)
Vậy....
b, \(\dfrac{4}{x}=\dfrac{7}{y}\Rightarrow\dfrac{x}{4}=\dfrac{y}{7}\)
Theo t/c dãy tỉ số bằng nhau ta có :
\(\dfrac{x}{4}=\dfrac{y}{7}=\dfrac{2x-y}{2.4-7}=\dfrac{10}{1}=10\)
\(\Rightarrow\left\{{}\begin{matrix}x=4.10=40\\y=7.10=70\end{matrix}\right.\)
Vậy......
c, Theo t/c dãy tỉ số bằng nhau ta có :
\(\dfrac{x}{4}=\dfrac{y}{6}=\dfrac{z}{9}=\dfrac{x-zy+z}{4-9.6+9}=\dfrac{-30}{-41}=\dfrac{30}{41}\)
\(\Rightarrow\left\{{}\begin{matrix}x=4.\dfrac{30}{41}=\dfrac{120}{41}\\y=6.\dfrac{30}{41}=\dfrac{180}{41}\\z=9.\dfrac{30}{41}=\dfrac{270}{41}\end{matrix}\right.\)
Vậy....
\(\dfrac{x+3}{5}=\dfrac{y-2}{3}=\dfrac{z-1}{7}\) Biết x-y+z = -86
Ta có : \(\dfrac{x+3}{5}=\dfrac{y-2}{3}=\dfrac{z-1}{7}=\dfrac{x+3-\left(y-2\right)+\left(z-1\right)}{5-3+7}=\dfrac{x+3-y+2+z-1}{9}=\dfrac{x-y+z+3+2-1}{9}=\dfrac{-86+4}{9}=\dfrac{-82}{9}\)
+) Với \(\dfrac{x+3}{5}=\dfrac{-82}{9}\)
=> x+3 = \(\dfrac{-82}{9}.5\) = \(\dfrac{-410}{9}\)
=> x = \(\dfrac{-410}{9}\) - 3 = \(\dfrac{-437}{9}\)
+) Với \(\dfrac{y-2}{3}=\dfrac{-82}{9}\)
=> y = \(\dfrac{-76}{3}\) ( Cách làm tương tự phần trên )
+) Với \(\dfrac{z-1}{7}=\dfrac{-82}{9}\)
=> z = \(\dfrac{-565}{9}\) ( Cách làm tương tự phần trên )
Vậy x = \(\dfrac{-437}{9}\)
y = \(\dfrac{-76}{3}\)
z = \(\dfrac{-565}{9}\)
P/s : Đảm bảo kết quả đúng 100% nếu như đề bài không sai 
Sửa đề x-y+z = -85 (số này đẹp hơn :))
Áp dụng t/c của dãy tỉ số = nhau có:
\(\dfrac{x+3}{5}=\dfrac{y-2}{3}=\dfrac{z-1}{7}=\dfrac{x+3-y+2+z-1}{5-3+7}=\dfrac{-85+4}{9}=-9\)
\(\Rightarrow\left\{{}\begin{matrix}x+3=-45\\y-2=-27\\z-1=-63\end{matrix}\right.\)\(\Rightarrow\left\{{}\begin{matrix}x=-48\\y=-25\\z=-62\end{matrix}\right.\)
Vậy...............
P/s: Nếu đề bạn đúng thì thay số vào thôi nhé!
1) Ta có :
\(\dfrac{1}{n}-\dfrac{1}{n+1}=\dfrac{n+1}{n\left(n+1\right)}-\dfrac{n}{n\left(n+1\right)}=\dfrac{1}{n\left(n+1\right)}\)
Vậy \(\dfrac{1}{n\left(n+1\right)}=\dfrac{1}{n}-\dfrac{1}{n+1}\rightarrowđpcm\)
2) \(A=\dfrac{1}{1.2}+\dfrac{1}{2.3}+............+\dfrac{1}{99.100}\)
\(\Leftrightarrow A=1-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+......+\dfrac{1}{99}-\dfrac{1}{100}\)
\(\Leftrightarrow A=1-\dfrac{1}{100}\)
\(\Leftrightarrow A=\dfrac{99}{100}\)
\(\dfrac{72-x}{7}=\dfrac{x-4}{9}\)
\(\Rightarrow9\left(72-x\right)=7\left(x-4\right)\)
\(\Rightarrow648-9x=2x-28\)
\(\Rightarrow11x-28=648\)
\(\Rightarrow11x=676\Rightarrow x=\dfrac{676}{11}\)
\(\dfrac{37-x}{x+13}=\dfrac{3}{7}\)
\(\Rightarrow7\left(37-x\right)=3\left(x+13\right)\)
\(\Rightarrow259-7x=3x+39\)
\(\Rightarrow10x+39=259\)
\(\Rightarrow10x=220\Rightarrow x=22\)
\(\dfrac{x+4}{20}=\dfrac{5}{x+4}\)
\(\Rightarrow\left(x+4\right)^2=100\)
\(\Rightarrow\left(x+4\right)^2=\pm10^2\)
\(\Rightarrow\left[{}\begin{matrix}x+4=10\Rightarrow x=6\\x+4=-10\Rightarrow x=-14\end{matrix}\right.\)
\(\dfrac{x-1}{x+2}=\dfrac{x-2}{x+3}\)
\(\Rightarrow\left(x-1\right)\left(x+3\right)=\left(x-2\right)\left(x+2\right)\)
\(\Rightarrow x\left(x+3\right)-1\left(x+3\right)=x\left(x+2\right)-2\left(x+2\right)\)
\(\Rightarrow x^2+3x-x-3=x^2+2x-2x-4\)
\(\Rightarrow x^2+2x-3=x^2-4\)
\(\Rightarrow2x-3=-4\)
\(\Rightarrow2x=-1\)
\(\Rightarrow x=-\dfrac{1}{2}\)
Chúc bạn học tốt
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