It's not the first one and what it does is illustrate completing the square. Using and proving are two different things, though. The 3-4-5 right triangle was widely used in what Herodotus called Ancient Egypt, but I'd be hella cautious about inferring they have done more than formulate it. It's an achievement, absolutely, just not as major.
The Sumerians/Babylonians/Mesopotamians/Akkadians/Etc get short shrift in history because (1) They were more of a continuous economic system with shifting centers of government across 3000 years than a monolithic culture (2) Their trade with the cultures Western Europe focuses on was constrained and largely unrecorded (3) Much of their culture was focused on keeping a Soviet-style socialist system humming along (4) Their style of writing and computation was kept deliberately complex so that only scribes could do it (5) Although they kept stringent records, they also switched from clay to paper about the time the barbarian tribes of the Mediterranean started picking up enough of their math to do doggerel versions of it so the records disappeared while the scribes became useless I read a now-forgotten sci fi book about a precocious kid who... was fundamentally boring, I don't remember much else but one of the "clever" things he did was point out how stupid it is to count in base 10, since it's only divisible once. He figured base 12 was substantially more useful. That really stuck with me. I have no idea why we don't learn that the Babylonians used base 12 for exactly that reason (they counted knuckle bones, not fingers, and they counted with their thumbs. So they divided the sky into 12 parts, found a constellation for each one, and doubled it for a day. Then they divided it into 12x5 and divided that again into 12x5 and here we still are, metric in everything but time. There are lots of blind spots in the history of science, culture and economics where if you stare at them long enough, Sargon of Akkad stares back at you.
I'm not exactly dissing on Sumerians here, just point out that it's one thing to use something and the other explaining the 'why it works' part. Just as Euclid was stunningly close to discovering complex and dual numbers 20-some centuries ahead of schedule, but didn't consider implications of particular x y and z, I have no problem believing Sumerians et al. could have missed it. Maybe they didn't think it needed to be proven, "since anyone can see it works." Maybe they found it but seen as aeolipile of applicable mathematics. Still, it'd be nice to see a tad more than 'just' application, since they're so goddamned close. Proofs are important, because math gets counter-intuitive very quickly and very easily. My most recently favorite example of that fact is Kempner series. Message brought to you by applied maths lobby.
I was discussing this graph with my cousin the other day: He observed how preposterous it was that we don't know for sure how the Pyramids were built but we bloody well know the interest rates of Babylon. I countered that 95% of the documents he had stored for safe keeping were receipts, warranties, tax returns and contracts, and why on earth would we suspect it's ever been different? I made a ritual of burning my homework and notes at the end of every quarter and the only books I kept were the ones I couldn't sell. And yet, because grown-ups tell you to, for years I had utility bills going back half a dozen years in case I needed to establish my address or some shit. The documents that get preserved are the documents that keep us from getting screwed over, or prove to someone else that they aren't being screwed. Much like money, literacy and math are most used to interact where there's no trust. We know about Gilgamesh because Ashurbanipal decided to aggrandize himself; we know how much beer workers got in 3000 BC because if they didn't get it nothing got built. Survivorship bias is a very real thing and the more advanced the culture, the more their relics favor H&R Block.
One thing to keep in mind is that base interest rate doesn’t matter as much as real interest rate. With modern monetary theory you really can’t compare pre 1971 and post 1971 rates without adjusting out inflation and you can’t really do that because at some point CPI and inflation decoupled due to and healthcare, housing weighting as well as hedonic adjustments there isn’t really a good number to use instead.