Wikipedia:Reference desk/Archives/Mathematics/2017 November 28

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November 28

Pascal's theorem

Theses Pascal's theorem proofs definitely need a visual presentation. And this proof overhere using Menelaus' theorem really lacks. יהודה שמחה ולדמן (talk) 09:19, 28 November 2017 (UTC)

Equation help

I was looking at a sample math equation for electrician aptitude test.

1. Consider the following formula: A = B + 3 ( 4 - C ) If B equals 5 and C equals 2, what is the value of A?

A.7 B.11 C.12 D.17

I get 10. The correct answer on the website says B. 11. Is 11 just an error?

CTF83! 22:23, 28 November 2017 (UTC)

I get 11. See Order of operations. Dolphin (t) 22:33, 28 November 2017 (UTC)

It works like this:

${\begin{aligned}A&=B+3(4-C)\\&=5+3(4-2)\\&=5+3\times 2\\&=5+6\\&=11\end{aligned}}$

--catslash (talk) 22:37, 28 November 2017 (UTC)

Your answer of 10 was the correct answer to A = ( B + 3 )( 4 - C ) Dbfirs 23:19, 28 November 2017 (UTC)

Did you intend A = B + 3 + ( 4 - C ) ? -- ToE 02:54, 29 November 2017 (UTC) Or are you working in hex?

Thanks for the answers. @Thinking of England:, I copied it directly from the practice test website, which appears to have an error. Would your example make the answer 11? CTF83! 09:08, 29 November 2017 (UTC)

Now I've gone and confused things!

• catslash, above, showed step-by-step (using the principles linked by Dolphin) the solution. Thus the website is correct. The problem, as you copied it, yields the answer of 1011, just like the website says.
• No insult intended (as the level of numeracy varies widely in the adult population), but this is a very simple problem. Anyone might make a simple error, but it seems surprising to us that when you saw what the correct answer was you didn't immediately see where the problem in your computation lay. Therefore we were trying to see how you came up with 10 in order to see if that revealed a simply misunderstanding we could help correct. I believe that is what Dbfirs was doing, however they made a simple mistake, either mathematical or just mistyping, and what they offered didn't yield the 10 you got.
• I was responding to Dbfirs, who suggested that ( B + 3 )( 4 - C ) = 10, when actually (5 + 3)(4 - 2) = 8 × 2 = 16. (And then I made a joke that 10 in hexadecimal (or base 16) is the same a 16 in decimal, though to make the distinction clear we would usually write it as 10₁₆ in mathematics or 0x10 in computer science.)
• I suggested that perhaps Dbfirs had intended to write ( B + 3 ) + ( 4 - C ) or B + 3 + ( 4 - C ) because 5 + 3 + (4 - 2) = 5 + 3 + 2 = 10.
• While my response was for Dbfirs, it was important for you as well. One way you could have come up with 10 was by misunderstanding (or simply misreading) the problem. If your eye added a + in the gap after the 3, then the new answer would be 10, but that is not how the notation works. Did you know that when two values are place next to one another, that is shorthand for indicating that they should be multiplied? Thus 3 A means 3 × A and 3 ( 4 - C ) means 3 × ( 4 - C ). So you are supposed to do what is inside the parentheses first, them multiply that result by 3, then add that result to B (as shown by catslash).
• So do you see what you had been doing wrong? Was it, perhaps, adding an implied addition were an implied multiplication really belongs?

I apologize for this wall of text, but I hope that straightens things out. -- ToE 14:05, 29 November 2017 (UTC)

Oh, the reference desk, what would we do without you? Thinking of England, I have corrected the typo in your first bullet point, I hope you don't mind. --JBL (talk) 16:52, 29 November 2017 (UTC)

Thanks. -- ToE 17:20, 29 November 2017 (UTC)

Sorry to cause confusion. I wasn't working in hex (though I used to do so), but I just mis-read the values late at night! Dbfirs 17:58, 29 November 2017 (UTC)

Ok I got it now, thanks all, esp Catslash! CTF83! 08:49, 30 November 2017 (UTC)