anonymous
  • anonymous
find an exact value of cos 165 degrees help :)?
Mathematics
  • Stacey Warren - Expert brainly.com
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SOLVED
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schrodinger
  • schrodinger
I got my questions answered at brainly.com in under 10 minutes. Go to brainly.com now for free help!
anonymous
  • anonymous
i think i might get it but not to sure
anonymous
  • anonymous
I got it! \[\cos(165) = \cos(120 + 45) = \cos(120)\cos(45) - \sin(120)\sin(45) \] \[= \frac{-1}{2} \frac{1}{\sqrt2}-\frac{\sqrt3}{2}\frac{1}{\sqrt2}\] Simply use: \[\cos(A \pm B) = \cos(A)\cos(B) \mp \sin(A)\sin(B)\]
anonymous
  • anonymous
\[-\sqrt{6}-\sqrt{2} \] over 4?

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phi
  • phi
you could also use the 1/2 angle formula cos(165) = -cos(15) = - cos(30/2)
anonymous
  • anonymous
i just don't get how to plug in to all this stuff you guys are giving me
phi
  • phi
but your value looks good. you can use a calculator and compare "decimal values" as a check
anonymous
  • anonymous
when my answer is on a form like i showed above the decimal i get is like.99 something how can that check it?
phi
  • phi
notice you solved cos(15) in your previous problem. you should be -0.96592586 make sure you are in degree mode (and not radian mode)
anonymous
  • anonymous
and i get .9659 for cos 15 but my answers look like this that i can pick from quantity square root of six plus square root of two divided by four. quantity negative square root of two plus square root of six divided by four. quantity negative square root of six minus square root of two divided by four. quantity negative square root of six plus one divided by four.
phi
  • phi
your answer up above is correct -(sqr(6)+sqr(2))/4 if you use your calculator, this expression will match -0.96592586
jdoe0001
  • jdoe0001
in short, do not use the calculator, add up the fractions only, leave it as rational
anonymous
  • anonymous
ok sweet thanks guys ok now heres were im confused i k now how to fin an exact value for cosine of nineteen pi divided by twelve.
phi
  • phi
It looks like they want you to use sums and differences formulas cos(A+B)= cos(A)cos(B)- sin(A)sin(B) and cos(A-B) = cos(A) cos(B)+ sin(A)sin(B)
phi
  • phi
I would change 19 pi/12 radians into degrees (I think better with degrees)
anonymous
  • anonymous
how do i know what to plug into the formula that you gave me?
phi
  • phi
First, find 19 pi/12 is 285 degees if you plot this, you see it is 15 degrees past 270 |dw:1378508905684:dw|
phi
  • phi
any ideas ?
anonymous
  • anonymous
quantity square root of six minus square root of two divided by four. that or quantity square root of two minus square root of six divided by four. that is what i think because of the unit circle
phi
  • phi
use cos(A+B)= cos(A)cos(B)- sin(A)sin(B)
anonymous
  • anonymous
so its the first one
phi
  • phi
I did not do the problem. But use A= 30 and B= 45 and use the formula
anonymous
  • anonymous
YES I GOT ALL OF THEM RIGHT THANK YOU SO MUCH FRIEND
anonymous
  • anonymous
if youre really bored i have more because my online teacher never wants to teach me -_- if not its cool
anonymous
  • anonymous
you a teacher or something?

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