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1; \(\dfrac{7}{15}\) + \(\dfrac{8}{15}\) = \(\dfrac{7+8}{15}\) = \(\dfrac{15}{15}\) = 1
2; \(\dfrac{1}{2}\) - \(\dfrac{1}{14}\) = \(\dfrac{1.7}{2.7}\) - \(\dfrac{1}{14}\) = \(\dfrac{7-1}{14}\) = \(\dfrac{6}{14}\) = \(\dfrac{3}{7}\)
3; \(\dfrac{8}{28}\) + \(\dfrac{-21}{35}\) = \(\dfrac{2}{7}\) + \(\dfrac{-21}{35}\)= \(\dfrac{10}{35}\) + \(\dfrac{-21}{35}\) = \(\dfrac{-11}{35}\)
4; \(\dfrac{3}{4}\) + \(\dfrac{2}{3}\) - \(\dfrac{9}{6}\) = \(\dfrac{9}{12}\) + \(\dfrac{8}{12}\) - \(\dfrac{18}{12}\) = \(\dfrac{9+8-18}{12}\) = \(\dfrac{-1}{12}\)
5; \(\dfrac{11}{36}\)- \(\dfrac{-7}{-24}\) = \(\dfrac{22}{72}\) + \(\dfrac{21}{72}\) = \(\dfrac{53}{72}\)
6; \(\dfrac{4}{15}\) + \(\dfrac{9}{5}\) - \(\dfrac{7}{3}\) = \(\dfrac{4}{15}\) + \(\dfrac{27}{15}\) - \(\dfrac{35}{15}\) = \(\dfrac{-4}{15}\)
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Bài 5 :
a, \(x+3=1\Leftrightarrow x=-2\)
b, \(19-x=-61\Leftrightarrow x=80\)
c, \(21-7x-12x+60=5\Leftrightarrow-19x=-76\Leftrightarrow x=4\)
d, \(\left(x-3\right)\left(x^2+1>0\right)=0\Leftrightarrow x=3\)
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\(A=3+3^2+...+3^{2005}\)
\(\Rightarrow3A=3^2+3^3+...+3^{2006}\)
\(\Rightarrow3A-A=3^{2006}-3\)
\(\Rightarrow2A=3^{2006}-3\)
\(\Rightarrow2A+3=3^{2006}\) là 1 lũy thừa của 3 (đpcm)
4.
\(B=1+1+2+2^2+2^3+...+2^{100}\)
\(2B=2+2+2^2+...+2^{101}\)
\(\Rightarrow2B-B=2+2^{101}-\left(1+1\right)=2^{101}\)
\(\Rightarrow B=2^{101}\) là 1 lũy thừa của 2 (đpcm)
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Bài 1:
\(A=2+2^2+2^3+...+2^{2003}+2^{2004}\)
\(=\left(2+2^2+2^3\right)+\left(2^4+2^5+2^6\right)+...+\left(2^{2002}+2^{2003}+2^{2004}\right)\)
\(=2\left(1+2+2^2\right)+2^4\left(1+2+2^2\right)+...+2^{2002}\left(1+2+2^2\right)\)
\(=7\cdot\left(2+2^4+...+2^{2002}\right)⋮7\)
Bài 2:
\(A=2+2^2+2^3+2^4+...+2^{59}+2^{60}\)
\(=\left(2+2^2+2^3+2^4\right)+\left(2^5+2^6+2^7+2^8\right)+...+\left(2^{57}+2^{58}+2^{59}+2^{60}\right)\)
\(=2\left(1+2+2^2+2^3\right)+2^5\left(1+2+2^2+2^3\right)+...+2^{57}\left(1+2+2^2+2^3\right)\)
\(=15\cdot\left(2+2^5+...+2^{57}\right)⋮15\)
Bài 3:
\(A=1+3+3^2+3^3+...+3^{1990}+3^{1991}\)
\(=\left(1+3+3^2\right)+\left(3^3+3^4+3^5\right)+...+\left(3^{1989}+3^{1990}+3^{1991}\right)\)
\(=13+3^3\left(1+3+3^2\right)+...+3^{1989}\left(1+3+3^2\right)\)
\(=13\left(1+3^3+...+3^{1989}\right)⋮13\)
Bài 4:
\(A=4+4^2+4^3+4^4+...+4^{23}+4^{24}\)
\(=\left(4+4^2\right)+\left(4^3+4^4\right)+...+\left(4^{23}+4^{24}\right)\)
\(=\left(4+4^2\right)+4^2\left(4+4^2\right)+...+4^{22}\left(4+4^2\right)\)
\(=20\left(1+4^2+...+4^{22}\right)⋮20\)
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a) \(\left(0,5\right)^{12}:\left(0,5\right)^{10}=\left(0,5\right)^{12-10}=\left(0,5\right)^2\)
b) \(\sqrt{36}=\pm6\)
c)\(\left(0,75\right)^{22}:\left(0,75\right)^{12}=\left(0,75\right)^{22-12}=\left(0,75\right)^{10}\)
d) \(\sqrt{49}=\pm7\)
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\(100:\left\{2\cdot\left[30-\left(12+7\right)\right]\right\}\)
\(=100:\left[2\cdot\left(30-19\right)\right]\)
\(=100:\left(2\cdot11\right)\)
\(=100:22\)
\(=\dfrac{100}{22}\)
\(=\dfrac{50}{11}\)
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