Should I reference the book I got a formula from, or the source that book got the formula from? I need to use a formula, found in a book &quot;A&quot;. The book itself refers to an academic paper from the 50s, &quot;B&quot;.
So when I use the formula, who should I reference?

We use the formula given as g = 4x^2 (&quot;A&quot;, 2001).
We use the formula given as  g = 4x^2 (&quot;B&quot;, 1950).
You should try to cite primary sources whenever possible; in this case, that means citing the original paper &quot;B&quot;.
However, as GEdgar says, a textbook may be an easier source for people to find, particularly if that book is the de facto standard text in your field. It is therefore a good idea (and perfectly acceptable) to cite both &quot;A&quot; and &quot;B&quot;, for example:

We use the formula given as g = 4x^2 (&quot;B&quot;, 1950, &quot;A&quot;, 2001).

Personally, I prefer to be more verbose and might write something along the lines of

The formula we use is given by Author B (&quot;B&quot;, 1950) as
g = 4x^2.
This was later used [confirmed/ expanded on/ usefully re-derived/ etc] by Author A in their book &quot;A&quot; (&quot;A&quot;, 2001).