When it comes to order of operations (PEMDAS, BODMAS, BEDMAS, BIDMAS), who are you going to believe: your primary school teacher, or Richard Feynman? Applying PEMDAS to the viral maths puzzle "6/2(1+2) = ?" gives the answer 9, as explained in videos by MindYourDecisions and others, but is that the whole story...

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References:
Abstract Algebra The Basic Graduate Year by Robert B. Ash https://faculty.math.illinois.edu/~r... p2
The Feynman Lectures on Physics Vol. I Ch. 6: Probability http://www.feynmanlectures.caltech.ed...
Basics of Mechanical Engineering by Paul D. Ronney http://ronney.usc.edu/ame101/ame101l... p7 (page 15 of the pdf)
Oliver Knill Ambiguous PEMDAS http://www.math.harvard.edu/~knill/pe...
AMS Guide for Reviewers May 2000 https://web.archive.org/web/200008152...
APS Physical Review Style and Notation Guide https://cdn.journals.aps.org/files/st... p21