In TC 24, Katja Kujanpää discusses the source of Paul’s quotation in Rom 11:35.
Typically, this is found in Job 41:3. But Kujanpää argues the source is actually a variant form of Isa 40:14.
According to the abstract,
Romans 11:35 is almost unanimously treated as a quotation from Job 41:3. Although it differs significantly from preserved Greek and Hebrew readings of that verse, few have questioned this attribution. In this article, I will argue that Rom 11:35 has nothing to do with Job but is a verbatim quotation from Isaiah. Scholars have mostly ignored the fact that Rom 11:35 agrees word for word with a Greek textual variant, a remarkably well attested plus in Isa 40:14. In the previous verse in Romans, Paul quotes Isa 40:13.
The essay draws attention to the importance of having as full as possible a sense of the Septuagintal textual tradition(s) when working on Paul’s (or the New Testament’s) use of Scripture.
It’s always possible that the source for a given New Testament quotation or allusion is represented in the apparatus rather than in the main text of a modern, critical Septuagint.
There are some cases where this possibility wouldn’t obtain (e.g., if a New Testament text has demonstrably influenced Septuagintal copies). But that doesn’t change the need to be on the look out for this possibility.
For the full essay, see TC 24’s webpage.