Question

Tìm các số a, b, c biết 2a= 3b, 5b= 7c và 3a- 7b+ 5c= -30

Answer 1

\(2a=3b\Rightarrow\dfrac{a}{3}=\dfrac{b}{2}\Rightarrow\dfrac{a}{21}=\dfrac{b}{14}\) (1)

\(5b=7c\Rightarrow\dfrac{b}{7}=\dfrac{c}{5}\Rightarrow\dfrac{b}{14}=\dfrac{c}{10}\) (2)

Từ (1), (2):

\(\Rightarrow\dfrac{a}{21}=\dfrac{b}{14}=\dfrac{c}{10}=\dfrac{3a}{63}=\dfrac{7b}{98}=\dfrac{5c}{50}=\dfrac{3a-7b+5c}{63-98+50}=\dfrac{-30}{15}=-2\)

\(\Rightarrow\left\{{}\begin{matrix}a=-2.21=-42\\b=-2.14=-28\\c=-2.10=-20\end{matrix}\right.\)