(ad + bc) = ((a + b) × (c + d)) – ac – bd.
First note this equation is more clearly be written as:
ad + bc = (a + b)(c + d) – ac – bd.
To see why this is so first expand (a + b)(c + d).
(a + b)(c + d) = ac + ad + bc + bd
now
(a + b)(c + d) − ac − bd = ac + ad + bc + bd − ac − bd
Hence
ad + bc = (a + b)(c + d) – ac – bd.
There are other weird formatting things in this article, which I blame on AI. I don't think the whole article was written by AI, but the copy-editing and formatting looks like an AI messed up things, such as those pointless round brackets or the inconsistency of multiplication (sometimes there is a × sign, sometimes there isn't), or this:
> have suspected that O(n²) was an inherent speed limit for multiplication. The celebrated Soviet math professor Andrey Kolmogorov posed the O(n^2)
The AI can't decide on notation.
It is sadly emblematic of how much SciAm has cheapened itself over the past decade. They used to care about technical details. Now they serve up poorly formatted rehashes of news (largely) from 65 years ago.