Question

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

A. 2
B. 5
C. 10
D. 15

Answer 1

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 =
`(x)/(f(x))` =
`(8)/(40)` =
`(1)/(5)`

thus x =
`(1)/(5)`f(x)

when x = 2

f(x) =
`(x)/(k)` =
`(2)/((1)/(5) )` = 10

Answer 2

f(x)=40

x=8

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