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

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

May 19, 2015

### Best Answer

#1
+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 = $$\underset{\,\,\,\,^{{360^\circ}}}{{cos}}^{\!\!\mathtt{-1}}{\left({\mathtt{\,-\,}}{\frac{{\mathtt{1}}}{{\mathtt{2}}}}\right)} = {\mathtt{120^{\circ}}}$$

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

May 19, 2015

### 2+0 Answers

#1
+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 = $$\underset{\,\,\,\,^{{360^\circ}}}{{cos}}^{\!\!\mathtt{-1}}{\left({\mathtt{\,-\,}}{\frac{{\mathtt{1}}}{{\mathtt{2}}}}\right)} = {\mathtt{120^{\circ}}}$$

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

kitty<3 May 19, 2015
#2
+112861
+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