\(a,\sqrt{\left(x-1\right)^2-\left(x^2-3\right)}=3\)

\(\Leftrightarrow\left(x-1\right)^2-\left(x^2-3\right)=9\)

\(\Leftrightarrow x^2-2x+1-x^2+3=9\)

\(\Leftrightarrow4-2x=9\)

\(\Leftrightarrow x=\dfrac{-5}{2}\)

\(b,\dfrac{x+3}{x}+\dfrac{x-3}{x-2}=2\)

\(\Leftrightarrow\dfrac{\left(x-3\right)\left(2x-2\right)}{x\left(x-2\right)}=2\)

\(\Leftrightarrow\left(x-3\right)\left(2x-2\right)=2x\left(x-2\right)\)

\(\Leftrightarrow2x^2-8x+6=2x^2-4x\)

\(\Leftrightarrow-4x=-6\)

\(\Leftrightarrow x=1,5\)

Gọi n_Mg = x 

       n_Al = y   (mol)

\(\left\{{}\begin{matrix}24x+27y=5,1\\x+1,5y=0,25\end{matrix}\right.\)

\(\rightarrow x=0,1;y=0,1\)

\(\%m_{Mg}=\dfrac{0,1.24}{5,1}.100\%\approx47,06\%\)

\(\%m_{Al}=100\%-47,06\%=52,94\%\)

\(m_{H_2SO_4}=\dfrac{\left(0,1+0,15\right).98.100}{10}=245\left(g\right)\)

Không thi nhưng sau vòng 3 cho mình xem đề 3 môn được k Phong, mình muốn tham khảo xíu:)

6. D

7. C

8. A

9. D

10. A

11. A

12. B

D

xuất hiện kết tủa trắng BaSO4

\(BaCl_2+H_2SO_4\rightarrow2HCl+BaSO_4\)