Question

Evaluate the trig expression cos120 degrees

Answer 1

\[\cos \left( 180-\theta \right)=-\cos \theta \]

Answer 2

hint: 120 degrees is in quadrant 2 (Q2)

the reference angle formula for Q2 is R = 180-x where x is the given angle and R is the reference angle

Answer 3

so 180-120?

Answer 4

yeah so the reference angle is 60 degrees

Answer 5

Is that it?

Answer 6

now evaluate cos(60)

Answer 7

-0.95

Answer 8

cos(120) = -cos(60)

keep in mind cosine is negative in Q2

Answer 9

make sure you are in degree mode

Answer 10

-0.0166?

Answer 11

no

Answer 12

Sorry if it's coming out weird. I don't have a scientific calculator (it broke) so I'm using the google calculator

Answer 13

type in "cos(60 degrees)" without quotes

Answer 14

0.5!

Answer 15

google defaults in radian mode so you have to explicitly tell it to be in degree mode

Answer 16

\[\cos 60=\frac{ 1 }{ 2 }\]

Answer 17

so your final answer should be -1/2

Answer 18

Ok :) Thanks!

Answer 19

yw