Question

Tìm x, biết:

a) \(\dfrac{x-3}{x+5}=\dfrac{5}{7}\)

b) \(\dfrac{x+4}{20}=\dfrac{5}{5x+4}\)

c)\(\dfrac{7}{x+1}=\dfrac{x+1}{9}\)

Answer 1

a, \(\dfrac{x-3}{x+5}=\dfrac{5}{7}\)

\(\Rightarrow7x-21=5x+25\)

\(\Rightarrow2x=46\)

\(\Rightarrow x=23\)

Vậy x = 23

b, \(\dfrac{x+4}{20}=\dfrac{5}{5x+4}\)

\(\Rightarrow\left(x+4\right)\left(5x+4\right)=100\)

\(\Rightarrow5x^2+24x+16=100\)

sai đề à?

c, \(\dfrac{7}{x+1}=\dfrac{x+1}{9}\)

\(\Rightarrow\left(x+1\right)^2=63\)

\(\Rightarrow\left[{}\begin{matrix}x+1=\sqrt{63}\\x+1=-\sqrt{63}\end{matrix}\right.\Rightarrow\left[{}\begin{matrix}x=\sqrt{63}-1\\x=-\sqrt{63}-1\end{matrix}\right.\)

Vậy...