Question

Find the value of sin minus 120 degree into cos 390 degree + cos minus 660 degree into sin 330 degree

Answer 1

Answer:

-1

Step-by-step explanation:

[sin (-120) * cos (390)] + [cos ( -660) * sin (330)]

sin ( -120 ) =  - sin 120  

                  =   - sin(90+30)

                  = - cos 30

cos 390 = cos ( 360+ 30)

             = cos 30

           

cos (-660) = cos ( 660 )

                  = cos ( 720-60 )

                   = cos 60 = sin 30

sin ( 330) = sin  ( 360 - 30 ) = - sin 30

the required answer is [ - cos 30* cos 30] +  [ - sin 30* sin 30]

                                     = - [ cos^2 30 + sin^2 30]

                                     = -1

therefore  its value is  -1 .