Proportionality?

[yo4561]

In the All the Quant Companion guide, there is a section on direct proportionality and inverse proportionality. From the explanation, it always seems that you will be either setting up a direct proportionality equation as (y1/x1)=(y2/x2) or inverse proportionality as (y1)(x1)=(y2)(x2). Is my understanding correct that these are the only two formulas you will need (I understand that you can express them in other ways, but would I be good to go if these are the ones in my toolkit)?

[esledge]

Your expressions are somewhat incomplete without a word explanation to accompany it. For example, (y1/x1)=(y2/x2) means that "y is directly proportional to x" and (y1)(x1)=(y2)(x2) means that "y is inversely proportional to x." (as opposed to x1 being proportional to x2, for example)

I personally like to memorize y = kx for direct and y = k/x for indirect proportionality between y and x, as they are cleaner and there's no need to juggle subscripts. This k also helps make the connection to the idea of the "unknown multiplier" in the Ratio part of All the Quant.

But yes, that's all you need for proportions!