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Cosine of pheta is equal to -1/2. Find pheta.

 May 19, 2015

Best Answer 

 #1
avatar+5479 
+20

You would use arccosine to solve this problem, since you have the cosine and need the angle:

 

cos(x) = -1/2

 

x = acos(-1/2)

 

x = $$\underset{\,\,\,\,^{\textcolor[rgb]{0.66,0.66,0.66}{360^\circ}}}{{cos}}^{\!\!\mathtt{-1}}{\left({\mathtt{\,-\,}}{\frac{{\mathtt{1}}}{{\mathtt{2}}}}\right)} = {\mathtt{120^{\circ}}}$$

 

* Which is also equal to 2pi/3 radians.

 May 19, 2015
 #1
avatar+5479 
+20
Best Answer

You would use arccosine to solve this problem, since you have the cosine and need the angle:

 

cos(x) = -1/2

 

x = acos(-1/2)

 

x = $$\underset{\,\,\,\,^{\textcolor[rgb]{0.66,0.66,0.66}{360^\circ}}}{{cos}}^{\!\!\mathtt{-1}}{\left({\mathtt{\,-\,}}{\frac{{\mathtt{1}}}{{\mathtt{2}}}}\right)} = {\mathtt{120^{\circ}}}$$

 

* Which is also equal to 2pi/3 radians.

kitty<3 May 19, 2015
 #2
avatar+112028 
+5

Kitty has found the 2nd quadrant answer.

there is also a third quadrant answer and that would be

180+60 = 240 degrees 

 

or

4pi/3 radians

 May 19, 2015

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