Question

If f(x) varies directly with x and f(x)=40 when x=8, find the value of f(x) when x=2.

Answer 1

f(x)=40

x=8

so if x=2 it would be 40/8 which is 5

Answer 2

C

since the quantities vary directly then

x = kf(x) ( where k is the constant of variation )

to find k use the given condition , f(x) = 40 when x = 2

k = $\frac{x}{f(x)}$ = $\frac{8}{40}$ = $\frac{1}{5}$

thus x = $\frac{1}{5}$f(x)

when x = 2

f(x) = $\frac{x}{k}$ = $\frac{2}{\frac{1}{5} }$ = 10