Question

If angle x is an acute angle and cos x = t, how do you express cos (180 - x) in terms of t?

Answer 1

Remember that `cos(A-B)=cosA*cosB+sinA*sinB` hence

#cos(180-x)=cos180*cosx+sinx*sin180=(-1)*cosx+sinx*0= -cosx=-t#

Finally `cos(180-x)=-t`