Where did I go wrong in this question? | Mathematics | Page 1

proud pine Apr 18, 2024, 3:53 AM

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I thought the answer was D, but apparently the correct answer is A?

peak cedarBOT Apr 18, 2024, 3:53 AM

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proud pine I thought the answer was D, but apparently the correct answer is A?

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devout copper Apr 18, 2024, 3:54 AM

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proud pine I thought the answer was D, but apparently the correct answer is A?

The derivative is wrong.

proud pine Apr 18, 2024, 3:55 AM

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why

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product rule, right?

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ohhh, wait

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it's supposed to be cos(x)² – sin(x)²

devout copper Apr 18, 2024, 3:56 AM

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Also known as cos(2x).

proud pine Apr 18, 2024, 3:57 AM

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wait what

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actually?

devout copper Apr 18, 2024, 3:57 AM

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...the double angle formula for cosine?

proud pine Apr 18, 2024, 3:57 AM

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i don't know what that is 💀

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i'm just a high school student studying calculus

devout copper Apr 18, 2024, 3:57 AM

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...right, which means you should've already studied trigonometry.

proud pine Apr 18, 2024, 3:58 AM

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devout copper Also known as `cos(2x)`.

that looks a bit similar to the power rule for logarithms

proud pine Apr 18, 2024, 3:58 AM

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devout copper ...right, which means you should've *already* studied trigonometry.

yes

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but we didn't study this

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is it important 💀

devout copper Apr 18, 2024, 3:58 AM

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...the double angle formulae are like some of the most basic things in trig.

proud pine Apr 18, 2024, 3:59 AM

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very nice

devout copper Apr 18, 2024, 3:59 AM

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Do you know the angle sum formulae?

proud pine Apr 18, 2024, 3:59 AM

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hmm...

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no 💀

devout copper Apr 18, 2024, 3:59 AM

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cos(a + b) = ?

proud pine Apr 18, 2024, 3:59 AM

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maybe i do know them but not what they're called

proud pine Apr 18, 2024, 3:59 AM

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devout copper `cos(a + b) = ?`

ah

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no

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or wait...

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is it cos(a) × cos(b) ?

devout copper Apr 18, 2024, 3:59 AM

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...no.

proud pine Apr 18, 2024, 4:00 AM

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damn

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:<

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what will it be then?

devout copper Apr 18, 2024, 4:06 AM

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cos(a + b) = cos(a)cos(b) - sin(a)sin(b).

proud pine Apr 18, 2024, 4:07 AM

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mmm...

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i see

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i'm studying AP Calculus AB, so maybe that's why i never heard of that

devout copper Apr 18, 2024, 4:08 AM

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...no, it's definitely not.

proud pine Apr 18, 2024, 4:08 AM

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and before that, i studied AP Precalculus

devout copper Apr 18, 2024, 4:08 AM

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...you learn trig in geometry.

proud pine Apr 18, 2024, 4:08 AM

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well, when did you learn about these formula ?

devout copper Apr 18, 2024, 4:09 AM

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...what do you mean, learn "about" them? I learned that they were true.

proud pine Apr 18, 2024, 4:09 AM

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aren't you a school graduate?

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you did learn about them in school?

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or by yourself?

devout copper Apr 18, 2024, 4:09 AM

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What do you think there is to learn "about" these formulae beyond their truth?

proud pine Apr 18, 2024, 4:10 AM

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thonk

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when did you know about these formulae and their undeniable proof?

devout copper Apr 18, 2024, 4:11 AM

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...are you asking me to prove the angle sum formula for cosine?

proud pine Apr 18, 2024, 4:11 AM

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no bruh

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💀

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i'm asking at what point in school did they teach you about these formulae

devout copper Apr 18, 2024, 4:12 AM

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I don't understand what you're asking me for. It's like you're asking me what I know about 2 + 2. I know it's 4.

devout copper Apr 18, 2024, 4:12 AM

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proud pine i'm asking at what point in school did they teach you about these formulae

In Geometry.

proud pine Apr 18, 2024, 4:12 AM

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which grade

devout copper Apr 18, 2024, 4:12 AM

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...ninth? I think?

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Wait, tenth.

proud pine Apr 18, 2024, 4:12 AM

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i see

devout copper Apr 18, 2024, 4:12 AM

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Ninth was algebra.

proud pine Apr 18, 2024, 4:12 AM

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we kinda just glossed over geometry 💀

#Where did I go wrong in this question?