Câu 16: A

Câu 14: C

Câu 12: A

I don't know 

Giải :

\(\dfrac{x+11}{89}+\dfrac{x+13}{87}-\dfrac{x+15}{85}-\dfrac{x+17}{83}=0\\ =>\left(\dfrac{x+11}{89}+1\right)+\left(\dfrac{x+13}{87}+1\right)-\left(\dfrac{x+15}{85}+1\right)-\left(\dfrac{x+17}{83}+1\right)=0\\ =>\left(\dfrac{x+11+89}{89}\right)+\dfrac{x+13+87}{87}-\dfrac{x+15+85}{85}-\dfrac{x+17+83}{83}=0\\ =>\dfrac{x+100}{89}+\dfrac{x+100}{87}-\dfrac{x+100}{85}-\dfrac{x+100}{83}=0\\ =>\left(x+100\right)\left(\dfrac{1}{89}+\dfrac{1}{87}-\dfrac{1}{85}-\dfrac{1}{83}\right)=0\\ =>\left[{}\begin{matrix}x+100=0\\\dfrac{1}{89}+\dfrac{1}{87}-\dfrac{1}{85}-\dfrac{1}{83}=0\left(voli\right)\end{matrix}\right.=>x=-100\)

  tìm x, y, z biết 

\(2x=3y\\ =>\dfrac{x}{3}=\dfrac{y}{2}\\ 4y=5z\\ =>\dfrac{y}{5}=\dfrac{z}{4}\\ \dfrac{x}{3}=\dfrac{y}{2}\\ =>\dfrac{x}{3.5}=\dfrac{y}{2.5}\\ =>\dfrac{x}{15}=\dfrac{y}{10}\\ \dfrac{y}{5}=\dfrac{z}{4}\\ =>\dfrac{y}{5.2}=\dfrac{z}{4.2}\\ =>\dfrac{y}{10}=\dfrac{z}{8}\\ =>\dfrac{x}{15}=\dfrac{y}{10}=\dfrac{z}{8}\)

Áp dụng tính chất dãy tỉ số bằng nhau

\(\dfrac{x}{15}=\dfrac{y}{10}=\dfrac{z}{8}=\dfrac{x+y+z}{15+10+8}=\dfrac{11}{33}=\dfrac{1}{3}\\ =>\left\{{}\begin{matrix}x=\dfrac{1}{3}.15=5\\y=\dfrac{1}{3}.10=\dfrac{10}{3}\\z=\dfrac{1}{3}.8=\dfrac{8}{3}\end{matrix}\right.\)

  tìm x, y, z biết

Giải

\(\dfrac{x}{3}=\dfrac{y}{4}\\ \Leftrightarrow\dfrac{x}{3.3}=\dfrac{y}{4.3}\\\Leftrightarrow\dfrac{x}{9}=\dfrac{y}{12}\\ \dfrac{y}{3}=\dfrac{z}{5}\\ \Leftrightarrow \dfrac{y}{3.4}=\dfrac{z}{5.4}\\ \Leftrightarrow\dfrac{y}{12}=\dfrac{z}{20}\\ =>\dfrac{x}{9}=\dfrac{y}{12}=\dfrac{z}{20}\)

Áp dụng tính chất dãy tỉ số bằng nhau 

\(\dfrac{x}{9}=\dfrac{y}{12}=\dfrac{z}{20}=\dfrac{2x+3x+z}{2.9+3.12+20}=\dfrac{6}{74}=\dfrac{3}{37}\\ \Rightarrow\left\{{}\begin{matrix}x=\dfrac{3}{37}\times9=\dfrac{27}{37}\\y=\dfrac{3}{37}\times12=\dfrac{36}{37}\\z=\dfrac{3}{37}\times20=\dfrac{60}{37}\end{matrix}\right.\)