Hello everyone!

This got me confused as I have not used proportionality for a very long time, all functions I deal with now are non-affine.

If a is proportional to b and a is prortional to d, which of the following is correct:

(1)$\displaystyle a = k_1 b$ and $\displaystyle a = k_2 d$ so $\displaystyle a = \frac{k_1}{2}b + \frac{k_2}{2}d$

(2) $\displaystyle a = k_1 b + k_2 d $

(3) $\displaystyle a = k_3 bd$

Thank you!