Question

Tìm x,y biết \(\dfrac{1-2y}{18}=\dfrac{1+4y}{24}=\dfrac{1+6y}{6x}\)

Tìm x,y,z biết \(\dfrac{x}{3}=\dfrac{y}{4}\) ; \(\dfrac{y}{5}=\dfrac{z}{7}\) và x+y+z =9^2

Tìm n để A= 12n^2 - 5n -25 là số nguyên tố

Answer 1

2)\(x+y+z=9^2=81\)

Ta có:\(\dfrac{x}{3}=\dfrac{y}{4}\Rightarrow\dfrac{x}{15}=\dfrac{y}{20}\left(1\right)\)

\(\dfrac{y}{5}=\dfrac{z}{7}\Rightarrow\dfrac{y}{20}=\dfrac{z}{28}\left(2\right)\)

Từ (1) và (2)\(\Rightarrow\dfrac{x}{15}=\dfrac{y}{20}=\dfrac{z}{28}\)

\(\Rightarrow\dfrac{x}{15}=\dfrac{y}{20}=\dfrac{z}{28}=\dfrac{x+y+z}{15+20+28}=\dfrac{81}{63}=\dfrac{9}{7}\)

\(\Rightarrow x=\dfrac{135}{7};y=\dfrac{180}{7};z=36\)