Math destroyer test 6 #7

[UFiA0389]

---

Members do not see this ad.

This is from MATH Destroyer 2010 Test 6 - Q7

Q - Which value is equal to cos (-120)

A. sin 60
B. cos 60
C. -sin 60
D. cos 30
E. -cos60

Destroyer says the answer is E but i thought in order to find a reference angle in the third quadrant you have to subtract -120-180 which equals 300. So why is it 60, do you ignore the negative sign on 120?

What am I missing...

 

[Tyrangiel]

.

 

[UFiA0389]

.

I cant see anything you wrote

 

[AlbinoPolarBear]

This is from MATH Destroyer 2010 Test 6 - Q7

Q - Which value is equal to cos (-120)

A. sin 60
B. cos 60
C. -sin 60
D. cos 30
E. -cos60

Destroyer says the answer is E but i thought in order to find a reference angle in the third quadrant you have to subtract -120-180 which equals 300. So why is it 60, do you ignore the negative sign on 120?

What am I missing...

cos (-120)
=cos (360-120)
= cos (240)

So it's in the 3rd quadrant, where cos is negative.
How far into 3rd quadrant? 240-180 = 60.

So 60 degrees into 3rd quadrant, with overall value being negative.

E. -cos(60)