Quantcast

A community for students. Sign up today!

Here's the question you clicked on:

55 members online
  • 0 replying
  • 0 viewing

lilsis76

  • 2 years ago

using addition formulas for sine and cosine to simplify the expression: cos (2pi/9) cos (pi/18) + sin (2pi/9) sin(pi/18) I want to learn it the normal way first before I add my instructors instructions to place S=45 & T = 30. here are the formulas given: sin(s+t) = sin s cos t + cos s sine t sin(s-t) = sin s cos t - cos s sine t cos(s+t) = cos s cos t - sin s sin t cos(s-t) = cos s cos t + sin s sin t The they way it looks, it seems to use formula 4: cos(s-t) = cos s cos t + sin s sin t

  • This Question is Closed
  1. dpaInc
    • 2 years ago
    Best Response
    You've already chosen the best response.
    Medals 1

    yes.... it is that 4th formula..

  2. lilsis76
    • 2 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    YAY! okay um... i know that know would i fill it in like this... well wait, first I have a question, how can i find out if its cos or sin that im looking for?

  3. lilsis76
    • 2 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    wait it would be the cos. right? ugh these formulas are confusing me

  4. dpaInc
    • 2 years ago
    Best Response
    You've already chosen the best response.
    Medals 1

    yes... the cosine one... just remember, the cosine formula (whether it's + or -), is cos * cos +/- sin * sin

  5. lilsis76
    • 2 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    okay, ill have to put that on a 3by5 card

  6. lilsis76
    • 2 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    okay! i think i got this just a second. let me know if its right or i made a mistake please.

  7. dpaInc
    • 2 years ago
    Best Response
    You've already chosen the best response.
    Medals 1

    so: \(\large cos(\frac{2\pi}{9}-\frac{\pi}{18})=cos(\frac{2\pi}{9})cos(\frac{\pi}{18})+sin(\frac{2\pi}{9})sin(\frac{\pi}{18}) \)

  8. lilsis76
    • 2 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Cos(2pi/9 – pi/18) mult. 2 to the left side……. So cos(4pi/18-pi/18) I guess giving me…….cos 3pi/18? Or do I simplify to get the cos pi/6 ???

  9. dpaInc
    • 2 years ago
    Best Response
    You've already chosen the best response.
    Medals 1

    now... \(\large \frac{2\pi}{9}-\frac{\pi}{18}=\frac{4\pi}{18}-\frac{\pi}{18}=\frac{3\pi}{18}=\frac{\pi}{6} \) that expression actually is cos(pi/6)

  10. dpaInc
    • 2 years ago
    Best Response
    You've already chosen the best response.
    Medals 1

    are you familiar with the unit circle?

  11. lilsis76
    • 2 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    yes, i had to memorize that thingy

  12. dpaInc
    • 2 years ago
    Best Response
    You've already chosen the best response.
    Medals 1

    wow... you seem like you don't need any help.... :)

  13. lilsis76
    • 2 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    so...my answer is going to be that unit?

  14. lilsis76
    • 2 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    ugh?! i do need h

  15. dpaInc
    • 2 years ago
    Best Response
    You've already chosen the best response.
    Medals 1

    so what's cos(pi/6) = ???

  16. lilsis76
    • 2 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    haha sorry about that. i just need help of steps walking through it, she went so fast in class. i only got pieces

  17. lilsis76
    • 2 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    30deg

  18. dpaInc
    • 2 years ago
    Best Response
    You've already chosen the best response.
    Medals 1

    yes... cos(pi/6) = cos(30 degrees) = ????

  19. lilsis76
    • 2 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    the points? is that what the ??? is for?

  20. dpaInc
    • 2 years ago
    Best Response
    You've already chosen the best response.
    Medals 1

    yes.... what's the x-coordinate ??? here is the unit circle...page 3 http://tutorial.math.lamar.edu/pdf/Trig_Cheat_Sheet.pdf

  21. lilsis76
    • 2 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    \[\sqrt{3}/2 , 1/2\]

  22. dpaInc
    • 2 years ago
    Best Response
    You've already chosen the best response.
    Medals 1

    i have to leave... sorry... the answer is cos(30 degrees) = \(\large \frac{\sqrt3}{2} \)

  23. lilsis76
    • 2 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    okay, thank u, bye!

  24. dpaInc
    • 2 years ago
    Best Response
    You've already chosen the best response.
    Medals 1

    the x-coordinate refers to the cosine, the y-coordinate is sine. yw... :)

  25. Not the answer you are looking for?
    Search for more explanations.

    • Attachments:

Ask your own question

Ask a Question
Find more explanations on OpenStudy

Your question is ready. Sign up for free to start getting answers.

spraguer (Moderator)
5 → View Detailed Profile

is replying to Can someone tell me what button the professor is hitting...

23

  • Teamwork 19 Teammate
  • Problem Solving 19 Hero
  • You have blocked this person.
  • ✔ You're a fan Checking fan status...

Thanks for being so helpful in mathematics. If you are getting quality help, make sure you spread the word about OpenStudy.

This is the testimonial you wrote.
You haven't written a testimonial for Owlfred.