anonymous
  • anonymous
Find an exact value. cos 165°
Mathematics
  • Stacey Warren - Expert brainly.com
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SOLVED
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chestercat
  • chestercat
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Nnesha
  • Nnesha
whats two number you can add to get 165? and those two number suppose to be on the unit circle
Nnesha
  • Nnesha
then use formula \[\large\rm cos(a+b)=\cos a \times \cos b - \sin a \times \sin b\]
anonymous
  • anonymous
90 & 75 ?

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Nnesha
  • Nnesha
well 75 is not on the unit circle :/
Nnesha
  • Nnesha
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anonymous
  • anonymous
120 + 45
Nnesha
  • Nnesha
30+135 40+120 225-60 for this u have to use cos(a-b)=cos a times cos b + sin a times sin b
Nnesha
  • Nnesha
perfect!
Nnesha
  • Nnesha
so \[\huge\rm cos(45+120)=\cos 45 \cos 120 - \sin 45 \sin 120\]
anonymous
  • anonymous
okay, i see. what do i do from there?
Nnesha
  • Nnesha
so \[\large\rm cos(45+120)=\cos 45 \cos 120 - \sin 45 \sin 120\] now use the unit circle cor exact value of cos and sin cos 45 =? cos 120 =? sin 45=? sin 120 =? plugin their exact values
anonymous
  • anonymous
cos 45 = (radical)2 / 2 cos 120 = -1/2 sin 45 = (radical) 2 /2 sin 120 = (radical) 3 /2 is that right?
Nnesha
  • Nnesha
\[\frac{ \sqrt{2} }{ 2 } \times \frac{ -1 }{ 2 } - \frac{ \sqrt{2} }{ 2 } \times \frac{ \sqrt{3} }{ 2}\] solve
Nnesha
  • Nnesha
yes that's right

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