Bài 14:
a, dễ
b, [TEX]{\color{Blue} \left ( \frac{1}{4} -1\right )\left ( \frac{1}{9}-1 \right )\left ( \frac{1}{16} -1 \right )....\left ( \frac{1}{81}-1 \right )\left ( \frac{1}{100} -1 \right )[/TEX]
[TEX]{\color{Blue} = \left ( \frac{-3}{4} \right )\left ( \frac{-8}{9} \right )\left ( \frac{-15}{16} \right )....\left ( \frac{-80}{81} \right )\left ( \frac{-99}{100} \right )[/TEX]
[TEX]{\color{Blue} =\left ( \frac{-1.3}{2^2} \right )\left ( \frac{-2.4}{3^2} \right )\left ( \frac{-3.5}{4^2} \right )....\left ( \frac{-8.10}{9^2} \right )\left ( \frac{-9.11}{10^2} \right )[/TEX]
[TEX]{\color{Blue} =\left ( \frac{-1}{2} \right )\left ( \frac{-11}{10} \right )=\frac{11}{20} [/TEX]
Bài 15:
Ta có: [TEX]{\color{Blue} \frac{x_1-1}{9}=\frac{x_2-2}{8}=........=\frac{x_9-9}{1}=\frac{x_1-1+x_2-2+...+x_9-9}{9+8+..+1}=\frac{90-45}{45}=1[/TEX]
[TEX]\Rightarrow {\color{Blue} x_1=x_2=x_3=...=x_9=10[/TEX]
Câu 10: 1,2 dễ
[TEX]3, \ {\color{Blue} \Leftrightarrow |x|-\frac{1}{8}=\left (\frac{-1}{8} \right )^7:\left ( \frac{-1}{8} \right )^5=\left ( \frac{-1}{8} \right )^2=\frac{1}{16}[/TEX]
[TEX]{\color{Blue} |x|=\frac{1}{16}+\frac{1}{8}=\frac{3}{16}[/TEX]
[TEX]{\color{Blue} \Leftrightarrow \left[\begin{x=\frac{3}{16}}\\{x=\frac{-3}{16}}[/TEX]
[TEX]4, \ {\color{Blue} 3x^2-6x=0 \Leftrightarrow 3x(x-2)=0 [/TEX]
[TEX]{\color{Blue} \Leftrightarrow \left[\begin{3x=0}\\{x-2=0}[/TEX] [TEX]{\color{Blue} \Leftrightarrow \left[\begin{x=0}\\{x=2}[/TEX]
Bài 11 dễ , tự làm
Bài 12: [TEX]{\color{Blue} 6xy-10x-3y=-4 \Leftrightarrow 6xy-10x-3y+4=0[/TEX]
[TEX]{\color{Blue} \Leftrightarrow 2(3xy-3y)-2(5x+2)=0 \Leftrightarrow 2(3xy-3y-5x-2)=0 \Rightarrow 3xy-3y-5x-2=0[/TEX]
[TEX]{\color{Blue} \Leftrightarrow 3y(x-1)-5(x-1)+3=0 \Leftrightarrow (x-1)(3y-5)=3[/TEX]
Đến đây chỉ xét x,y thôi 
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