\(=5x.2x^2-5x.3x-5x.1\)

\($\\$\)

\(=10x^3-15x^2-5x\)

\(a.\) Gọi khối lượng niken, kẽm, đồng lần lượt là \(a,b,c\left(kg\right)\left(a,b,c>0\right)\)

Áp dụng tính chất dãy tỉ số bằng nhau :

\(\dfrac{a}{3}=\dfrac{b}{4}=\dfrac{c}{13}=\dfrac{a+b+c}{3+4+13}=\dfrac{150}{20}=\dfrac{15}{2}\)

\(\Rightarrow\) ^{\(\left[{}\begin{matrix}a=\dfrac{15}{2}.3=\dfrac{45}{2}\\b=\dfrac{15}{2}.4=\dfrac{60}{2}=30\\c=\dfrac{15}{2}.13=\dfrac{195}{2}\end{matrix}\right.\)}

^\(b.\) \(\dfrac{x+1}{3}=\dfrac{y+2}{4}=\dfrac{z-1}{5}=\dfrac{3x+3}{9}=\dfrac{3x+3-y-2+z-1}{9-4+5}=\dfrac{50}{10}=5\)

\(\Rightarrow\) \(\left[{}\begin{matrix}x+1=15\\y+2=20\end{matrix}\right.\) \(\Rightarrow\) \(\left[{}\begin{matrix}x=14\\y=18\end{matrix}\right.\)

\(b.\)

\(\Rightarrow\) \(x=\dfrac{-3.8}{2}=\dfrac{-24}{2}=-12\)

\(=\dfrac{-1}{2}-\dfrac{1}{2}+\dfrac{-3}{2}+1=\left[\left(\dfrac{-1}{2}+\dfrac{-1}{2}\right)+1\right]+\dfrac{-3}{2}=\dfrac{-3}{2}\)

\(1\) D

\(2 \) D

39) B

40) D