Precalculus, Chapter 4, 4.4, Section 4.4, Problem 57

1)
sin(-840^@)
As we know that
sin(-x) = -sin(x)
so ,
sin(-840^@) = -sin(840^@)
so ,
-sin(840^@)= -sin(2*360^@+120^@) = -sin(120^@)

= -sin(180 - 60) = -sin(60) = - sqrt(3)/ 2
2)

cos(-840^@)
As we know that
cos(-x) = cos(x)
so ,
cos(-840^@) = cos(840^@)
so ,
cos(840^@)= cos(2*360^@+120^@) = cos(120^@)

= cos(180 - 60) = -cos(60) = - 1/ 2
3)
Tan(-840^@)= sin(-840^@)/(cos(-840^@)) = (- sqrt(3)/ 2)/(- 1/ 2) = sqrt(3)