# Math Help - Inversely proportional

1. ## Inversely proportional

Have I done this correctly?

f is inversely proportional to d

when d=50, f=256
Find value of f when d=80

Figured out a constant of 25/125

so

f=50/1*125/25=250

Is thsi correct?

2. its incorrect
since f is inversely proportional to d we have
f=k/d where k is a constant
now as f=256 when d=50 then
k=fd=50*256
so value of f when d=80
f=(50*256)/80=160

3. f is inversely proportional to d ...

$f = \frac{k}{d}$

... where k is the constant of proportionality.

substitute in the given values to solve for k ...

$256 = \frac{k}{50}$

$k = 50 \cdot 256 = 12800
$

$f = \frac{12800}{d}$

... finish the problem

4. ah I did d/k.

Thanks.