\((1)\\ f(x)=30x\ \ \ \Leftrightarrow\ \ \ \frac{f(x)}{x} =30\ \ \ \Rightarrow\ \ \ constant=30\ \ \ \Rightarrow\ \ \ Ans.\ B.\\\\(2)\\consatnt= \frac{y}{x} = \frac{4}{1} =4\ \ \ \Rightarrow\ \ \ Ans.\ D.\\\\(3)\\f(x)=m\cdot x\ \ \ and\ \ \ f(6)=8\\\\m\cdot6=8\ \ \ \Rightarrow\ \ \ m= \frac{8}{6} = \frac{4}{3} \ \ \ \Rightarrow\ \ \ y= \frac{4}{3}x\ \ \ \ Ans.\ (?)\)
\((4)\\f(x)=m\cdot x\ \ and\ \ f(8)=56\\\\m\cdot8=56\ \ \Leftrightarrow\ \ m= \frac{56}{8} =7\ \ \Rightarrow\ \ \ f(x)=7x\ \ \ \Rightarrow\ \ \ f(2)=7\cdot2=14\\\\Ans.\ D.\\\\(5)\\direct:\ \ y=m\cdot x\ \ \ and\ \ \ m\in R\\e.g.:\ \ y=2x,\ \ y=0.5x,\ \ y= \frac{x}{7}= \frac{1}{7}x,\ ...\\\\NOT\ direct:\ \ \ e.g.:\ \ y= \sqrt{x} ,\ \ y= \frac{1}{x},\ \ y=x^2,\ ...\)
