math destroyer question

[bipster10]

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For those with math destroyer 2010 edition, this is #7 on practice test 6.
For all others here is the question.

Which value is equal to cos(-120)?
a. sin60
b. cos60
c. -sin60
d. cos30
e. -cos60

See I understand the entire reference angle thing as well as the CAST method, but I don't understand why it is e and not c. Because it is negative, we go in the clockwise direction and that leads me into the III quadrant, or the T in CAST. From what I understand that means only tangent is positive. So how do I know that it is -cos60 and not -sin60.

Any help appreciated!

 

[cm8a12321]

For those with math destroyer 2010 edition, this is #7 on practice test 6.
For all others here is the question.

Which value is equal to cos(-120)?
a. sin60
b. cos60
c. -sin60
d. cos30
e. -cos60

See I understand the entire reference angle thing as well as the CAST method, but I don't understand why it is e and not c. Because it is negative, we go in the clockwise direction and that leads me into the III quadrant, or the T in CAST. From what I understand that means only tangent is positive. So how do I know that it is -cos60 and not -sin60.

Any help appreciated!

120 degree will be in the second quadrant. In the second quadrant, all sine terms are positive, where cosine and tan are negative.

Note: Cos (-A)=cosA; sin(-A)=-sinA

So, in order to find cos (-120), you need to find the reference angle of 120, which is (180-120=60).

so the now the term is -cos(60), taking into account that 120 degree is in quadrant II.