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If f(x) varies directly with x and f(x)=-70 when x=-10 what is f(x) when x=5

User Jeff Glass
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1 Answer

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\bf \begin{array}{cccccclllll} \textit{something}&&\textit{varies directly to}&&\textit{something else}\\ \quad \\ \textit{something}&=&{{ \textit{some value}}}&\cdot &\textit{something else}\\ \quad \\ f(x)&=&{{ k}}&\cdot&x && \boxed{f(x)={{ k }}x} \end{array} \\\\ \textit{now, we know that } \begin{cases} f(x)=-70\\ x=-10 \end{cases}\implies \boxed{-70=k(-10)}

solve for "k", to get the "constant of variation",
and plug it back in f(x) = kx

what's f(x) when x = 5? just make "x" to 5 to get f(x)
User Sayannayas
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