Bài 3:

\(A=75\left(4^{2004}+4^{2003}+...+4^2+4+1\right)+25\)

Đặt: \(B=4^{2004}+4^{2003}+...+4^2+4+1\)

\(4B=4^{2005}+4^{2004}+...+4^3+4^2+4\\ 4B-B=\left(4^{2005}+4^{2004}+...+4^3+4^2+4\right)-\left(4^{2004}+4^{2003}+...+4^2+4+1\right)\\ 3B=4^{2005}-1\\ B=\dfrac{4^{2005}-1}{3}\)

\(=>A=75\cdot\dfrac{4^{2005}-1}{3}+25\\ =25\left(4^{2005}-1\right)+25\\ =25\cdot\left(4^{2005}-1+1\right)\\ =25\cdot4^{2005}\\ =25\cdot4\cdot4^{2004}\\ =100\cdot4^{2004}\) 

=> A chia hết cho 100 

Bài 1:

a: \(\dfrac{5\cdot3^{11}+4\cdot3^{17}}{3^9\cdot5^2-3^9\cdot2^3}=\dfrac{3^{11}\cdot\left(5+4\cdot3^6\right)}{3^9\left(5^2-2^3\right)}\)

\(=3^2\cdot\dfrac{5+4\cdot729}{25-8}=3^2\cdot\dfrac{2921}{17}=\dfrac{26289}{17}\)

b: \(\dfrac{1}{2\cdot5}+\dfrac{1}{5\cdot8}+...+\dfrac{1}{47\cdot50}\)

\(=\dfrac{1}{3}\left(\dfrac{3}{2\cdot5}+\dfrac{3}{5\cdot8}+...+\dfrac{3}{47\cdot50}\right)\)

\(=\dfrac{1}{3}\left(\dfrac{1}{2}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{8}+...+\dfrac{1}{47}-\dfrac{1}{50}\right)\)

\(=\dfrac{1}{3}\left(\dfrac{1}{2}-\dfrac{1}{50}\right)=\dfrac{1}{3}\cdot\dfrac{24}{50}=\dfrac{24}{150}=\dfrac{8}{50}=\dfrac{4}{25}\)

c: \(1+\dfrac{2}{6}+\dfrac{2}{12}+...+\dfrac{2}{9900}\)

\(=\dfrac{2}{2}+\dfrac{2}{6}+...+\dfrac{2}{9900}\)

\(=2\left(\dfrac{1}{2}+\dfrac{1}{6}+...+\dfrac{1}{9900}\right)\)

\(=2\left(\dfrac{1}{1\cdot2}+\dfrac{1}{2\cdot3}+...+\dfrac{1}{99\cdot100}\right)\)

\(=2\left(1-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+...+\dfrac{1}{99}-\dfrac{1}{100}\right)\)

\(=2\left(1-\dfrac{1}{100}\right)=2\cdot\dfrac{99}{100}=\dfrac{99}{50}\)

d: \(\left(\dfrac{1}{2^2}-1\right)\left(\dfrac{1}{3^2}-1\right)\cdot...\cdot\left(\dfrac{1}{99^2}-1\right)\)

\(=\left(\dfrac{1}{2}-1\right)\left(\dfrac{1}{3}-1\right)\cdot...\cdot\left(\dfrac{1}{99}-1\right)\left(\dfrac{1}{2}+1\right)\cdot\left(\dfrac{1}{3}+1\right)\cdot...\cdot\left(\dfrac{1}{99}+1\right)\)

\(=\dfrac{-1}{2}\cdot\dfrac{-2}{3}\cdot...\cdot\dfrac{-98}{99}\cdot\dfrac{3}{2}\cdot\dfrac{4}{3}\cdot...\cdot\dfrac{100}{99}\)

\(=\dfrac{1}{99}\cdot\dfrac{100}{2}=\dfrac{50}{99}\)

\(a)3^{2x-1}+2\cdot9^{x-1}=405\\ =>3^{2x-1}+2\cdot\left(3^2\right)^{x-1}=405\\ =>3^{2x-1}+2\cdot3^{2x-2}=405\\ =>3^{2x-2}\cdot\left(3+2\right)=405\\ =>3^{2x-2}\cdot5=405\\ =>3^{2x-2}=\dfrac{405}{5}=81\\ =>3^{2x-2}=3^4\\ =>2x-2=4\\ =>2x=4+2=6\\ =>x=\dfrac{6}{2}\\ =>x=3\)

\(b)\left(\dfrac{1}{3}\right)^{x-1}+5\left(\dfrac{1}{3}\right)^{x+1}=\dfrac{14}{9^3}\\ =>\left(\dfrac{1}{3}\right)^{x-1}\left(1+5\cdot\dfrac{1}{3^2}\right)=\dfrac{14}{729}\\ =>\left(\dfrac{1}{3}\right)^{x-1}\cdot\dfrac{14}{9}=\dfrac{14}{729}\\ =>\left(\dfrac{1}{3}\right)^{x-1}=\dfrac{14}{729}:\dfrac{14}{9}\\ =>\left(\dfrac{1}{3}\right)^{x-1}=\dfrac{9}{729}=\dfrac{1}{81}\\ =>\left(\dfrac{1}{3}\right)^{x-1}=\left(\dfrac{1}{3}\right)^4\\ =>x-1=4\\ =>x=1+4\\ =>x=5\)

\(c)\dfrac{3}{5}\left(3x^3-\dfrac{8}{9}\right)-\dfrac{1}{2}\left(\dfrac{3}{2}-1\right)=-\dfrac{1}{4}\\ =>\dfrac{3}{5}\left(3x^3-\dfrac{8}{9}\right)-\dfrac{1}{2}\cdot\dfrac{1}{2}=-\dfrac{1}{4}\\ =>\dfrac{3}{5}\left(3x^3-\dfrac{8}{9}\right)-\dfrac{1}{4}=-\dfrac{1}{4}\\ =>\dfrac{3}{5}\left(3x^3-\dfrac{8}{9}\right)=0\\ =>3x^3-\dfrac{8}{9}=0\\ =>3x^3=\dfrac{8}{9}\\ =>x^3=\dfrac{8}{9}:3=\dfrac{8}{27}\\ =>x^3=\left(\dfrac{2}{3}\right)^3\\ =>x=\dfrac{2}{3}\)

a: \(3^{2x-1}+2\cdot9^{x-1}=405\)

=>\(\dfrac{3^{2x}}{3}+2\cdot3^{2x-2}=405\)

=>\(\dfrac{1}{3}\cdot3^{2x}+2\cdot3^{2x}\cdot\dfrac{1}{9}=405\)

=>\(3^{2x}\cdot\left(\dfrac{1}{3}+\dfrac{2}{9}\right)=405\)

=>\(3^{2x}\cdot\dfrac{5}{9}=405\)

=>\(3^{2x}=405:\dfrac{5}{9}=405\cdot\dfrac{9}{5}=81\cdot9=3^6\)

=>2x=6

=>x=3

b: \(\left(\dfrac{1}{3}\right)^{x-1}+5\cdot\left(\dfrac{1}{3}\right)^{x+1}=\dfrac{14}{9^3}\)

=>\(\left(\dfrac{1}{3}\right)^x\cdot3+5\cdot\left(\dfrac{1}{3}\right)^x\cdot\dfrac{1}{3}=\dfrac{14}{9^3}\)

=>\(\left(\dfrac{1}{3}\right)^x\cdot\left(3+\dfrac{5}{3}\right)=\dfrac{14}{9^3}\)

=>\(\left(\dfrac{1}{3}\right)^x=\dfrac{14}{3^6}:\dfrac{14}{3}=\dfrac{3}{3^6}=\dfrac{1}{3^5}\)

=>x=5

c: \(\dfrac{3}{5}\left(3x^3-\dfrac{8}{9}\right)-\dfrac{1}{2}\left(\dfrac{3}{2}-1\right)=-\dfrac{1}{4}\)

=>\(\dfrac{9}{5}x^3-\dfrac{24}{45}-\dfrac{1}{2}\cdot\dfrac{1}{2}+\dfrac{1}{4}=0\)

=>\(\dfrac{9}{5}x^3=\dfrac{24}{45}=\dfrac{8}{15}\)

=>\(x^3=\dfrac{8}{15}:\dfrac{9}{5}=\dfrac{8}{15}\cdot\dfrac{5}{9}=\dfrac{40}{135}=\dfrac{8}{27}=\left(\dfrac{2}{3}\right)^3\)

=>\(x=\dfrac{2}{3}\)

d: \(\dfrac{7}{x}+\dfrac{4}{5\cdot9}+\dfrac{4}{9\cdot13}+...+\dfrac{4}{41\cdot45}=\dfrac{29}{45}\)

=>\(\dfrac{7}{x}+\dfrac{1}{5}-\dfrac{1}{9}+\dfrac{1}{9}-\dfrac{1}{13}+...+\dfrac{1}{41}-\dfrac{1}{45}=\dfrac{29}{45}\)

=>\(\dfrac{7}{x}+\dfrac{9}{45}-\dfrac{1}{45}=\dfrac{29}{45}\)

=>\(\dfrac{7}{x}=\dfrac{29}{45}-\dfrac{8}{45}=\dfrac{21}{45}=\dfrac{7}{15}\)

=>x=15

e: \(\dfrac{1}{3\cdot5}+\dfrac{1}{5\cdot7}+...+\dfrac{1}{\left(2x+1\right)\left(2x+3\right)}=\dfrac{5}{31}\)

=>\(\dfrac{2}{3\cdot5}+\dfrac{2}{5\cdot7}+...+\dfrac{2}{\left(2x+1\right)\left(2x+3\right)}=\dfrac{10}{31}\)

=>\(\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{7}+...+\dfrac{1}{2x+1}-\dfrac{1}{2x+3}=\dfrac{10}{31}\)

=>\(\dfrac{1}{3}-\dfrac{1}{2x+3}=\dfrac{10}{31}\)

=>\(\dfrac{1}{2x+3}=\dfrac{1}{3}-\dfrac{10}{31}=\dfrac{1}{93}\)

=>2x+3=93

=>2x=90

=>x=45

a; (2\(x\) - 1)² = 49

     (2\(x\) - 1)² = 7²

     \(\left[{}\begin{matrix}2x-1=-7\\2x-1=7\end{matrix}\right.\)

       \(\left[{}\begin{matrix}2x=-7+1\\2x=7+1\end{matrix}\right.\)

       \(\left[{}\begin{matrix}2x=-6\\2x=8\end{matrix}\right.\)

         \(\left[{}\begin{matrix}x=-\dfrac{6}{2}\\x=\dfrac{8}{2}\end{matrix}\right.\)

        \(\left[{}\begin{matrix}x=-3\\x=4\end{matrix}\right.\)

Vậy \(x\) \(\in\) {- 3; 4} 

b;  (5\(x-3\))² - (4\(x\) - 7)² = 0

    (5\(x\) - 3 - 4\(x\) + 7)(5\(x\) - 3 + 4\(x\) - 7) = 0

   (\(x\) + 4)(9\(x\) - 10) = 0

    \(\left[{}\begin{matrix}x+4=0\\9x-10=0\end{matrix}\right.\)

     \(\left[{}\begin{matrix}x=-4\\x=\dfrac{10}{9}\end{matrix}\right.\)

Vậy  \(x\) \(\in\) {- 4; \(\dfrac{10}{9}\)}

b: \(\dfrac{2}{5}-\left(\dfrac{4}{3}+\dfrac{4}{5}\right)-\left(-\dfrac{1}{9}-0,4\right)+\dfrac{11}{9}\)

\(=\dfrac{2}{5}-\dfrac{4}{3}-\dfrac{4}{5}+\dfrac{1}{9}+\dfrac{2}{5}+\dfrac{11}{9}\)

\(=\left(\dfrac{2}{5}-\dfrac{4}{5}+\dfrac{2}{5}\right)+\left(-\dfrac{4}{3}+\dfrac{1}{9}+\dfrac{11}{9}\right)\)

\(=-\dfrac{4}{3}+\dfrac{12}{9}=0\)

c: \(\dfrac{11}{8}\cdot\left[\left(-\dfrac{5}{11}:\dfrac{13}{8}-\dfrac{5}{11}:\dfrac{13}{5}\right)+\dfrac{-6}{33}\right]+\dfrac{3}{4}\)

\(=\dfrac{11}{8}\cdot\left[-\dfrac{5}{11}\cdot\dfrac{8}{13}-\dfrac{5}{11}\cdot\dfrac{5}{13}+\dfrac{-2}{11}\right]+\dfrac{3}{4}\)

\(=\dfrac{11}{8}\cdot\left[-\dfrac{5}{11}\left(\dfrac{8}{13}+\dfrac{5}{13}\right)-\dfrac{2}{11}\right]+\dfrac{3}{4}\)

\(=\dfrac{11}{8}\cdot\dfrac{-7}{11}+\dfrac{3}{4}=-\dfrac{7}{8}+\dfrac{3}{4}=-\dfrac{1}{8}\)

a:\(\widehat{BAC}+\widehat{xAC}=180^0\)(hai góc kề bù)

=> \(\widehat{BAC}+70^0=180^0\)

=>\(\widehat{BAC}=110^0\)

Ta có: \(\widehat{BAC}+\widehat{ABD}=180^0\)

mà hai góc này là hai góc ở vị trí trong cùng phía

nên AC//BD

b: Vì AC//BD

nên \(\widehat{yCx}=\widehat{CDB}\)(hai góc đồng vị)

=>\(\widehat{yCx}=60^0\)

Ta có: \(\widehat{yCx}+\widehat{ACD}=180^0\)(hai góc kề bù)

=>\(\widehat{ACD}+60^0=180^0\)

=>\(\widehat{ACD}=120^0\)

Ta có: \(\widehat{BAC}+\widehat{ABD}=180^0\)(AC//BD)

=>\(\widehat{BAC}+70^0=180^0\)

=>\(\widehat{BAC}=110^0\)

e: \(\dfrac{x+1}{10}+\dfrac{x+1}{11}+\dfrac{x+1}{12}=\dfrac{x+1}{13}+\dfrac{x+1}{14}\)

=>\(\left(x+1\right)\left(\dfrac{1}{10}+\dfrac{1}{11}+\dfrac{1}{12}\right)=\left(x+1\right)\left(\dfrac{1}{13}+\dfrac{1}{14}\right)\)

=>\(\left(x+1\right)\left(\dfrac{1}{10}+\dfrac{1}{11}+\dfrac{1}{12}-\dfrac{1}{13}-\dfrac{1}{14}\right)=0\)

=>x+1=0

=>x=-1

\(\left\{{}\begin{matrix}3x-4y=-2\\5x+2y=14\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}3x-4y=-2\\2y=14-5x\end{matrix}\right.\)

=>\(\left\{{}\begin{matrix}3x-2\cdot2y=-2\\2y=14-5x\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}3x-2\left(14-5x\right)=-2\\2y=14-5x\end{matrix}\right.\)

=>\(\left\{{}\begin{matrix}3x-28+10x=-2\\2y=-5x+14\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}13x=-2+28=26\\2y=-5x+14\end{matrix}\right.\)

=>\(\left\{{}\begin{matrix}x=2\\2y=-5\cdot2+14=14-10=4\end{matrix}\right.\)

=>\(\left\{{}\begin{matrix}x=2\\y=2\end{matrix}\right.\)

Bài 8:

\(4)-\dfrac{20}{7}:\dfrac{5}{21}\\ =-\dfrac{20}{7}\cdot\dfrac{21}{5}\\ =-4\cdot3\\ =-12\\ 5)-\dfrac{8}{5}:\dfrac{-12}{7}\\ =\dfrac{-8}{5}\cdot\dfrac{-7}{12}\\ =\dfrac{14}{15}\\ 6)\dfrac{-12}{21}:\dfrac{1}{6}\\ =\dfrac{-4}{7}\cdot6\\ =-\dfrac{24}{7}\)

Bài 10:

1: \(4,5\cdot\left(-\dfrac{4}{9}\right)=-\dfrac{9}{2}\cdot\dfrac{4}{9}=-\dfrac{4}{2}=-2\)

2: \(2,4\cdot\left(-3\dfrac{4}{7}\right)=\dfrac{-12}{5}\cdot\dfrac{25}{7}=\dfrac{-12}{7}\cdot\dfrac{25}{5}=-5\cdot\dfrac{12}{7}=-\dfrac{60}{7}\)

3: \(0,2\cdot\dfrac{-15}{4}=\dfrac{1}{5}\cdot\dfrac{-15}{4}=-\dfrac{15}{5}\cdot\dfrac{1}{4}=-\dfrac{3}{4}\)

4: \(\left(-3,5\right):\left(-2\dfrac{4}{5}\right)=\dfrac{3.5}{2\dfrac{4}{5}}=\dfrac{3.5}{2.8}=\dfrac{5}{4}\)

5: \(\dfrac{-5}{23}:\left(-2\right)=\dfrac{5}{23}:2=\dfrac{5}{23\cdot2}=\dfrac{5}{46}\)

6: \(1,25:\left(-3\dfrac{1}{8}\right)=1.25:\dfrac{-25}{8}=1.25\cdot\dfrac{-8}{25}=-\dfrac{10}{25}=-\dfrac{2}{5}\)

Bài 9:

1: \(-3\dfrac{1}{9}\cdot\dfrac{4}{21}=\dfrac{-28}{9}\cdot\dfrac{4}{21}=-\dfrac{28}{21}\cdot\dfrac{4}{9}=-\dfrac{4}{9}\cdot\dfrac{4}{3}=-\dfrac{16}{27}\)

2: \(-\dfrac{3}{4}\cdot2\dfrac{1}{2}=-\dfrac{3}{4}\cdot\dfrac{5}{2}=\dfrac{-15}{8}\)

3: \(-\dfrac{8}{15}\cdot1\dfrac{1}{4}=-\dfrac{8}{15}\cdot\dfrac{5}{4}=\dfrac{-40}{60}=-\dfrac{2}{3}\)

4: \(-\dfrac{11}{15}:1\dfrac{1}{10}=-\dfrac{11}{15}:\dfrac{11}{10}=-\dfrac{11}{15}\cdot\dfrac{10}{11}=-\dfrac{10}{15}=-\dfrac{2}{3}\)

5: \(1\dfrac{1}{5}:\left(-2\dfrac{1}{5}\right)=\dfrac{6}{5}:\dfrac{-11}{5}=\dfrac{6}{5}\cdot\dfrac{5}{-11}=\dfrac{6}{-11}=-\dfrac{6}{11}\)

6: \(\left(-3\dfrac{1}{7}\right):\left(-1\dfrac{6}{49}\right)=\dfrac{-22}{7}:\dfrac{-55}{49}=\dfrac{22}{7}\cdot\dfrac{49}{55}\)

\(=\dfrac{22}{55}\cdot\dfrac{49}{7}=7\cdot\dfrac{2}{5}=\dfrac{14}{5}\)

\(6)\left(2x+1\right)^2-\left(x+3\right)^2=0\\ \Leftrightarrow\left(2x+1-x-3\right)\left(2x+1+x+3\right)=0\\ \Leftrightarrow\left(x-2\right)\left(3x+4\right)=0\\ \Leftrightarrow\left[{}\begin{matrix}x-2=0\\3x+4=0\end{matrix}\right.\\ \Leftrightarrow\left[{}\begin{matrix}x=2\\x=-\dfrac{4}{3}\end{matrix}\right.\\ 7)\left(x^2-4\right)+x\left(x-2\right)=0\\ \Leftrightarrow\left(x-2\right)\left(x+2\right)+x\left(x-2\right)=0\\ \Leftrightarrow\left(x-2\right)\left(x+2+x\right)=0\\ \Leftrightarrow\left(x-2\right)\left(2x+2\right)=0\\ \Leftrightarrow\left[{}\begin{matrix}x-2=0\\2x+2=0\end{matrix}\right.\\ \Leftrightarrow\left[{}\begin{matrix}x=2\\x=-1\end{matrix}\right.\\ 8)2\left(x+1\right)=\left(5x-1\right)\left(x+1\right)\\ \Leftrightarrow2\left(x+1\right)-\left(5x-1\right)\left(x+1\right)=0\\ \Leftrightarrow\left(x+1\right)\left(2-5x+1\right)=0\\ \Leftrightarrow\left(x+1\right)\left(3-5x\right)=0\\ \Leftrightarrow\left[{}\begin{matrix}x=-1\\x=\dfrac{3}{5}\end{matrix}\right.\\ 9)\left(-4x+3\right)x=\left(2x+5\right)x\\ \Leftrightarrow\left(-4x+3\right)x-\left(2x+5\right)x=0\\ \Leftrightarrow x\left(-4x+3-2x-5\right)=0\\ \Leftrightarrow x\left(-6x-2\right)=0\\ \Leftrightarrow\left[{}\begin{matrix}x=0\\x=-\dfrac{1}{3}\end{matrix}\right.\)