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=( 23./x-1/+32).57=59 =(8./x-1/+9)=59:57 = 8./x-1/ +9=25 =8./x-1/=25-9 8./x-1/=16 /x-1/=16:8 /x-1/=2 x=2+1 x=3
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\(\Rightarrow\left(21n+18\right)⋮\left(3n-2\right)\\ \Rightarrow\left(21n-14+32\right)⋮\left(3n-2\right)\\ \Rightarrow\left[7n\left(3n-2\right)+32\right]⋮\left(3n-2\right)\\ \Rightarrow3n-2\inƯ\left(32\right)=\left\{1;2;4;8;16;32\right\}\\ \Rightarrow3n\in\left\{3;4;6;10;18;34\right\}\\ \Rightarrow n\in\left\{1;2;6\right\}\left(n\in N\right)\)
\(\Leftrightarrow3n-2\in\left\{1;-1;2;-2;4;-4;8;-8;16;-16;32;-32\right\}\)
hay \(n\in\left\{1;0;2;6\right\}\)
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1.2.3...9 - 1.2.3...8 - 1.2.3...82
= 1.2.3...8.(9 - 1) - 1.2.3...82
= 1.2.3...8.8 - 1.2.3...8.8
= 0
1.2.3...9 - 1.2.3...8 - 1.2.3...82
= 1.2.3...8.(9 - 1) - 1.2.3...82
= 1.2.3...8.8 - 1.2.3...8.8
= 0
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\(a,36:\left(x-5\right)=2^2\\ 36:\left(x-5\right)=4\\ x-5=9\\ x=14\)
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2/1.3+2/3.5+...+2/x(x+2)= 40/41
1-1/3+1/3-1/5+...+1/x-1/(x+2)=40/41
1-1/(x+2)=40/41
1/(x+2)=1-40/41=1/41
x+2=41
x=41-2=39
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Ta có: \(x-\frac{20}{11\cdot13}-\frac{20}{13\cdot15}-...-\frac{20}{53\cdot55}=\frac{3}{11}\)
\(\Leftrightarrow x-10\cdot\left(\frac{2}{11\cdot13}+\frac{2}{13\cdot15}+...+\frac{2}{53\cdot55}\right)=\frac{3}{11}\)
\(\Leftrightarrow x-10\cdot\left(\frac{1}{11}-\frac{1}{13}+\frac{1}{13}-\frac{1}{15}+...+\frac{1}{53}-\frac{1}{55}\right)=\frac{3}{11}\)
\(\Leftrightarrow x-10\cdot\left(\frac{1}{11}-\frac{1}{55}\right)=\frac{3}{11}\)
\(\Leftrightarrow x-10\cdot\frac{4}{55}=\frac{3}{11}\)
\(\Leftrightarrow x-\frac{8}{11}=\frac{3}{11}\)
\(\Leftrightarrow x=\frac{3}{11}+\frac{8}{11}\)
\(\Leftrightarrow x=1\)
Vậy \(x=1\)thỏa mãn đề.
![](https://rs.olm.vn/images/avt/0.png?1311)
tớ cũng học bồi dưỡng ,,k minh nha
giải
ta có:B=1/1x2x3+1/2x3x4+... +1/18x19x220
=>2B=2/1x2x3+2/2x3x4+...2/18x19x20
=(1/1x2-1/2x3)+(1/2x3-1/3x4)+..+(1/18x19-1/19x20)
=1/1x2-1/19x20=189/380
=>B=189/760<1/4
kb đi có gì tớ giải cho
![](https://rs.olm.vn/images/avt/0.png?1311)
Giải:
a) \(2\dfrac{17}{20}-1\dfrac{15}{11}+6\dfrac{9}{20}:3\)
\(=\dfrac{57}{20}-\dfrac{26}{11}+\dfrac{129}{20}:3\)
\(=\dfrac{107}{220}+\dfrac{43}{20}\)
\(=\dfrac{29}{11}\)
b) \(4\dfrac{3}{7}:\left(\dfrac{7}{5}.4\dfrac{3}{7}\right)\)
\(=\dfrac{31}{7}:\left(\dfrac{7}{5}.\dfrac{31}{7}\right)\)
\(=\dfrac{31}{7}:\dfrac{31}{5}\)
\(=\dfrac{5}{7}\)
c) \(\left(3\dfrac{2}{9}.\dfrac{15}{23}.1\dfrac{7}{29}\right):\dfrac{5}{23}\)
\(=\left(\dfrac{29}{9}.\dfrac{15}{23}.\dfrac{36}{29}\right):\dfrac{5}{23}\)
\(=\dfrac{60}{23}:\dfrac{5}{23}\)
\(=12\)
![](https://rs.olm.vn/images/avt/0.png?1311)
a) \(\dfrac{13}{20}+\dfrac{3}{5}+x=\dfrac{5}{6}\)
\(\Rightarrow\dfrac{5}{4}+x=\dfrac{5}{6}\)
\(\Rightarrow x=\dfrac{5}{6}-\dfrac{5}{4}\)
\(\Rightarrow x=\dfrac{-5}{12}\)
b) \(x+\dfrac{1}{3}=\dfrac{2}{5}-\dfrac{-1}{3}\)
\(\Rightarrow x+\dfrac{1}{3}=\dfrac{11}{15}\)
\(\Rightarrow x=\dfrac{11}{15}-\dfrac{1}{3}\)
\(\Rightarrow x=\dfrac{2}{5}\)
c)\(\dfrac{-5}{8}-x=\dfrac{-3}{20}-\dfrac{-1}{6}\)
\(\dfrac{-5}{8}-x=\dfrac{1}{60}\)
\(\Rightarrow x=\dfrac{-5}{8}-\dfrac{1}{60}\)
\(\Rightarrow x=\dfrac{-77}{120}\)
d) \(\dfrac{3}{5}-x=\dfrac{1}{4}+\dfrac{7}{10}\)
\(\Rightarrow\dfrac{3}{5}-x=\dfrac{19}{20}\)
\(\Rightarrow x=\dfrac{3}{5}-\dfrac{19}{20}\)
\(\Rightarrow x=\dfrac{-7}{20}\)
e) \(\dfrac{-3}{7}-x=\dfrac{4}{5}+\dfrac{-2}{3}\)
\(\Rightarrow\dfrac{-3}{7}-x=\dfrac{2}{15}\)
\(\Rightarrow x=\dfrac{-3}{7}-\dfrac{2}{15}\)
\(\Rightarrow x=\dfrac{-59}{105}\)
g) \(\dfrac{-5}{6}-x=\dfrac{7}{12}+\dfrac{-1}{3}\)
\(\Rightarrow\dfrac{-5}{6}-x=\dfrac{1}{4}\)
\(\Rightarrow x=\dfrac{-5}{6}-\dfrac{1}{4}\)
\(\Rightarrow x=\dfrac{-13}{12}\)