Asked • 12/14/19

Solve cos x = -2/3 for 0 ≤ x < 2π (round to the nearest hundredth of a radian)

I received the values 131.81 and 228.209 which is incorrect. I've tried to come up with the correct answer multiple times. Solve cos x = -2/3 for 0 ≤ x < 2π (round to the nearest hundredth of a radian). Thanks for your help!

Mark M.

The measure your give are in degrees (and correct), yet the problem asks for radians.
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12/14/19

Carla J.

131.81 is 2.3 in radians and 228.209 is 3.9 in radians. How would I know where they are located in the four quadrants, would I use a trig circle?
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12/14/19

1 Expert Answer

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Mark H. answered • 12/14/19

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This was posted earlier--and agrees with what another tutor posted:


A negative cosine means the angle is in either the 2nd or 3rd quadrant, where x is negative.


With a cosine of 2/3, the basic angle is 0.841 radians (+ or -)

Since we have -2/3, the angle is pi ± 0.841


If this is not what you are being given, then tell us what it's supposed to be, and we can maybe figure out the disconnect.


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