# Thread: Need help yet again, direct proportionality

1. ## Need help yet again, direct proportionality

Hi, this time its:
Q is directly proportional to the square of R

Q = 3 when R= 4

a)Find a formula for Q in terms of R

b)Work out the value of Q when R= 8

Thanks
Matt

2. First, find k, your constant of proportionality.

$Q=kR^{2}$

Use the initial conditions they gave you to find k.

$3=k(4)^{2}=16k$

$k=\frac{3}{16}$

$Q=\frac{3}{16}(8)^{2}$