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Có : 10A = 10^15-10/10^15-11 = (10^15-11)+1/10^15-11 = 1 + 1/10^15-11
10B = 10^15+10/10^15+9 = (10^15+9)+1/10^15+9 = 1 + 1/10^15+9
Vì 10^15-11 < 10^15-9 => 1/10^15-11 > 1/10^15+9 => 10A > 10B
=> A < B
k mk nha
![](https://rs.olm.vn/images/avt/0.png?1311)
Bài 1 : Ta thấy
\(\dfrac{10}{15}=\dfrac{2}{3};\dfrac{14}{21}=\dfrac{2}{3}\Rightarrow10:15=14:21\Rightarrow\dfrac{10}{15}=\dfrac{14}{21}\)
\(\dfrac{16}{\left(-4\right)}=-4;\dfrac{12}{\left(-3\right)}=-4\Rightarrow16:\left(-4\right)=12:\left(-3\right)\Rightarrow\dfrac{16}{\left(-4\right)}=\dfrac{12}{\left(-3\right)}=-4\)
\(\dfrac{\left(-5\right)}{15}=\dfrac{\left(-1,2\right)}{3,6}=-\dfrac{1}{3}\Rightarrow\left(-5\right):15=\left(-1,2\right):3,6\)
\(\dfrac{2}{3}:\dfrac{1}{4}=\dfrac{2}{3}.4=\dfrac{8}{3};\dfrac{16}{9}:\dfrac{16}{24}=\dfrac{16}{9}.\dfrac{24}{16}=\dfrac{8}{3}\)
\(\Rightarrow\left(\dfrac{2}{3}:\dfrac{1}{4}\right)=\left(\dfrac{16}{9}:\dfrac{16}{24}\right)=\dfrac{8}{3}\)
Bài 2 :
a) \(14.15=10.21\Rightarrow\dfrac{14}{10}=\dfrac{21}{15}=\dfrac{7}{5}\)
b) \(0,2.4,5=0,6.1,5\Rightarrow\dfrac{0,2}{0,6}=\dfrac{1,5}{4,5}=\dfrac{1}{3}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
Câu 2:
\(B=\dfrac{5^{21}\cdot\left(2\cdot5-9\right)}{5^{20}}\cdot\dfrac{7^{15}\left(7+3\right)}{15\cdot7^{15}-95\cdot7^{14}}\)
\(=\dfrac{5\cdot1}{1}\cdot\dfrac{7^{15}\cdot10}{7^{14}\cdot\left(15\cdot7-95\right)}\)
\(=5\cdot\dfrac{7\cdot10}{105-95}=5\cdot7=35\)
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\(\dfrac{x+1}{10}+\dfrac{x+1}{11}+\dfrac{x+1}{12}+\dfrac{x+1}{13}=\dfrac{x+1}{14}+\dfrac{x+1}{15}\)
\(\Leftrightarrow\left(x+1\right)\left(\dfrac{1}{10}+\dfrac{1}{11}+\dfrac{1}{12}+\dfrac{1}{13}\right)=\left(x+1\right)\left(\dfrac{1}{14}+\dfrac{1}{15}\right)\)
\(\Leftrightarrow\left(x+1\right)\left(\dfrac{1}{10}+\dfrac{1}{11}+\dfrac{1}{12}-\dfrac{1}{13}-\dfrac{1}{14}\right)=0\)
\(\Leftrightarrow x+1=0\)
\(\Leftrightarrow x=-1\)
\(\dfrac{x+1}{10}+\dfrac{x+1}{11}+\dfrac{x+1}{12}+\dfrac{x+1}{13}=\dfrac{x+1}{14}+\dfrac{x+1}{15}\)
<=> \(\dfrac{x+1}{10}+\dfrac{x+1}{11}+\dfrac{x+1}{12}+\dfrac{x+1}{13}-\dfrac{x+1}{14}-\dfrac{x+1}{15}=0\)
<=> \(\left(x+1\right)\left(\dfrac{1}{10}+\dfrac{1}{11}+\dfrac{1}{12}+\dfrac{1}{13}-\dfrac{1}{14}-\dfrac{1}{15}\right)=0\)
Do: \(\dfrac{1}{10}+\dfrac{1}{11}+\dfrac{1}{12}+\dfrac{1}{13}-\dfrac{1}{14}-\dfrac{1}{14}>0\) nên x + 1 = 0
Vậy x = -1
![](https://rs.olm.vn/images/avt/0.png?1311)
Lời giải:
Ta có:
\(\frac{1}{13}; \frac{1}{14}; \frac{1}{15}<\frac{1}{12}\)
\(\Rightarrow \frac{1}{13}+\frac{1}{14}+\frac{1}{15}< \frac{3}{12}=\frac{1}{4}\)
\(\frac{1}{61}; \frac{1}{62};\frac{1}{63}< \frac{1}{60}\)
\(\Rightarrow \frac{1}{61}+\frac{1}{62}+\frac{1}{63}< \frac{3}{60}=\frac{1}{20}\)
Do đó:
\(A< \frac{1}{5}+\frac{1}{4}+\frac{1}{20}=\frac{9}{20}+\frac{1}{20}\)
\(\Leftrightarrow A< \frac{1}{2}\) (đpcm)
Đặt biểu thức bằng A:
\(\Rightarrow A=\dfrac{1}{5}\left(\dfrac{1}{13}+\dfrac{1}{14}+\dfrac{1}{15}\right)+\left(\dfrac{1}{61}+\dfrac{1}{62}+\dfrac{1}{63}\right)\)
Ta thấy: \(\dfrac{1}{13}+\dfrac{1}{14}+\dfrac{1}{15}< 3.\dfrac{1}{61}\)
\(\dfrac{1}{61}+\dfrac{1}{62}+\dfrac{1}{63}< 3.\dfrac{1}{61}\)
\(\Rightarrow A< \dfrac{1}{5}+\dfrac{3}{31}+\dfrac{3}{61}< \dfrac{1}{2}\left(đpcm\right)\)
![](https://rs.olm.vn/images/avt/0.png?1311)
a) \(\dfrac{-12}{15}+\dfrac{-4}{26}=\dfrac{-4}{5}+\dfrac{-2}{13}=\dfrac{-52-10}{65}=\dfrac{-62}{65}\)
b) \(5\dfrac{1}{3}-2\dfrac{4}{5}=\dfrac{16}{3}-\dfrac{14}{5}=\dfrac{80}{15}-\dfrac{42}{15}=\dfrac{38}{15}\)
c) \(\dfrac{4}{5}-\left(-\dfrac{2}{7}\right)+\dfrac{-5}{10}=\dfrac{4}{5}+\dfrac{2}{7}-\dfrac{1}{2}=\dfrac{56}{70}+\dfrac{20}{70}-\dfrac{35}{70}=\dfrac{41}{70}\)
d) \(-1\dfrac{2}{7}+\dfrac{3}{14}-\dfrac{5}{21}=\dfrac{-9}{7}+\dfrac{3}{14}-\dfrac{5}{21}=\dfrac{-54}{42}+\dfrac{9}{42}-\dfrac{10}{42}=\dfrac{-55}{42}\)
e) \(12-\dfrac{11}{121}+\left(\dfrac{-8}{9}\right)-\left(-\dfrac{3}{7}\right)\)
\(=12-\dfrac{11}{121}-\dfrac{8}{9}+\dfrac{3}{7}\)
\(=\dfrac{91476}{7623}-\dfrac{693}{7623}-\dfrac{6776}{7623}+\dfrac{3267}{7623}\)
\(=\dfrac{7934}{693}\)
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\(1.\dfrac{27^4.4^3}{9^5.8^2}=\dfrac{3^{12}.2^6}{3^{10}.2^6}=3^2=9\)
\(2.\dfrac{8^5.3^{15}}{2^{14}.81^4}=\dfrac{2^{15}.3^{15}}{2^{14}.3^{16}}=\dfrac{2}{3}\)
Ý 1:
\(\dfrac{27^4.4^3}{9^5.8^2}=\dfrac{\left(3^3\right)^4.\left(2^2\right)^3}{\left(3^2\right)^5.\left(2^3\right)^2}=\dfrac{3^{12}.2^6}{3^{10}.2^6}=3^2=9\)
Ý 2:
\(\dfrac{8^5.3^{15}}{2^{14}.81^4}=\dfrac{\left(2^3\right)^5.3^{15}}{2^{14}.\left(3^4\right)^4}=\dfrac{2^{15}.3^{15}}{2^{14}.3^{16}}=\dfrac{2^{14}.2.3^{15}}{2^{14}.3^{15}.3}=\dfrac{2}{3}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
\(\dfrac{x-3}{13}+\dfrac{x-3}{14}=\dfrac{x-3}{15}+\dfrac{x-3}{16}\)
\(\Leftrightarrow\dfrac{1680.\left(x-3\right)+1560.\left(x-3\right)-1456.\left(x-3\right)-1365.\left(x-3\right)}{21840}=0\)
\(\Leftrightarrow\left(x-3\right).\left(1680+1560-1456-1365\right)=0\)
\(\Leftrightarrow\left(x-3\right).419=0\)
\(\Leftrightarrow419x=1257\)
\(\Leftrightarrow x=3\)
Lời giải:
\(\frac{x-3}{13}+\frac{x-3}{14}=\frac{x-3}{15}+\frac{x-3}{16}\)
\((x-3)\left(\frac{1}{13}+\frac{1}{14}\right)=(x-3)\left(\frac{1}{15}+\frac{1}{16}\right)\)
\((x-3)\left[\left(\frac{1}{13}+\frac{1}{14}\right)-\left(\frac{1}{15}+\frac{1}{16}\right)\right]=0\)
Ta thấy:
\(\frac{1}{13}>\frac{1}{15}; \frac{1}{14}>\frac{1}{16}\Rightarrow \frac{1}{13}+\frac{1}{14}> \frac{1}{15}+\frac{1}{16}\)
Do đó biểu thức trong ngoặc vuông lớn hơn $0$ hay khác $0$
$\Rightarrow x-3=0$
$\Leftrightarrow x=3$
\(A=\dfrac{14^{14}+1}{14^{15}+1}\)
\(\Rightarrow14.A=\dfrac{14^{15}+14}{14^{15}+1}\)
\(\Rightarrow14.A=\dfrac{14^{15}+1}{14^{15}+1}+\dfrac{13}{14^{15}+1}\)
\(\Rightarrow14.A=1+\dfrac{13}{14^{15}+1}\)
\(B=\dfrac{14^{15}+1}{14^{16}+1}\)
\(\Rightarrow14.B=\dfrac{14^{16}+14}{14^{16}+1}\)
\(\Rightarrow14.B=\dfrac{14^{16}+1}{14^{16}+1}+\dfrac{13}{14^{16}+1}\)
\(\Rightarrow14.B=1+\dfrac{13}{14^{16}+1}\)
Nhận xét: \(\dfrac{13}{14^{15}+1}>\dfrac{13}{14^{16}+1}\) (cùng tử, xét mẫu)
\(\Rightarrow A>B\)
Vậy \(A>B\)