Addition formulae - Trig help requested

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Homework Help Overview

The discussion revolves around the application of trigonometric addition formulas, specifically in relation to the angles 120 degrees and 165 degrees. The original poster expresses confusion regarding the calculation of sin(120) and cos(120) within the context of the sine addition formula.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning

Approaches and Questions Raised

  • The original poster attempts to apply the sine addition formula but struggles with the values of sin(120) and cos(120). Some participants suggest reviewing the addition formula and the unit circle for clarification. Others question the correctness of the original poster's application of the formula.

Discussion Status

Participants are actively engaging in clarifying the sine addition formula and the fundamental angles related to 120 degrees. There is a focus on correcting misunderstandings and exploring the properties of sine and cosine for specific angles, though no consensus has been reached on the original poster's approach.

Contextual Notes

The original poster has indicated a specific numerical result that they do not understand, which may suggest a misunderstanding of the addition formula or the values of the trigonometric functions involved. There is also a mention of the unit circle as a potential resource for understanding the angles.

kalistella
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addition formulae - Trig help requested!

I have a problem with this with regard to Sin 120 or Cos 120.

Eg

sin 165
sin (120 +45)
sin120.cos45-cos120.sin45

How do I deal with sin 120 or cos 120?

I know that cos 45 is 1/square root 2 and so is sin 45.

But in the above problem, the answer comes to 1/4 (square root 6 - square root 2)

I don't get this answer.:rolleyes: My problem is with the 120 angles.
Please help!

Thanks!
 
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Well, learn the correct addition formula properly.
Then, look at the unit circle what sin(120) and cos(120) should be.
 
kalistella said:
sin (120 +45)
sin120.cos45-cos120.sin45
This is wrong. Check your addition formula again.

marlon
 
As arildno and marlon have already pointed out, you have the sine sum formula wrong! As for 120 degrees what fundamental angles do you know sine and cosine for? (You seem to have 45 degrees okay.) Would it help to remember that 120= 60+ 60 or that 120= 90+ 30?
 

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