Note (check mark notwithstanding) that the guest’s answer is absolute bullshit.
This was posted by the forum’s Wrong Answer Troll (WAT), who occasionally posts work product to give the appearance of legitimacy to his wrong answers.
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The equation $\dfrac{a+b}{a-3b}=5$ is a line with a slope of $m= \dfrac{b}{a} \rightarrow \dfrac{1}{4}$
The inverse of this slope is the ratio of $\dfrac{a}{b} \rightarrow \dfrac{4}{1}$
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I’ve followed WAT for almost two years. A few weeks ago I zeroed in on a very likely suspect from my short list. (I can’t prove it, though). He’s a long-time member who periodically posts reasonably competent solutions to moderately advanced questions. There are two related sources for his primary motivation in creating these wrong answer posts. I’m not going to elaborate beyond adding that this derivation is based on the parameters of the question posts he trolls.
This forum, like the world at large, is full of fascinating people. Most human-equations are probably not fully solvable, but close approximations are available statistically, with reasonable confidence intervals.
One open question is how he will react to this post, assuming he sees it. There is always a reaction; it’s not always obvious. 
GA
+118667 I think guest just made a careless mistake that is commonly made Ginger.
I do not think this error was intentional,.
I do not think this error was intentional.
You may be right. The solution answer of one (1) was so obviously wrong it seemed intentional. I just dropped it into WAT’s box of Wrong Answers with Work Product and incremented the counter.
WAT’s usual fair is to plop the name of a theorem or formula, which may or may not (usually not) relate to the math in the question, followed by a simple answer. Here’s a recent example:
By Cauchy-Schwarz, the maximum value is 1/n^2.
WAT didn’t start his trolling with “name dropping,” he just plopped wrong answers. Lately, he’s morphed into giving work product, but he has yet to “name drop” when he does this –probably because the solution methods do not have an associated name or the name is obvious, such as the Pythagorean Theorem.
This is a fascinating case study and a recreational diversion from weightier responsibilities. 
GA
