Determine if two variables are directly proportional. if so, write the equation of variation.
Volume of a sphere (V) and radius (R).
v is proportional to r is
v = kr
Since v = 4/3 πr^3, v is proportional to r^3, but not r.
Easy to check:
v(1) = 4π/3
v(2) = 32π/3
v(2)/v(1) = 32/4 ≠ 2/1