Use an Addition or Subtraction Formula to write the expressi

Use an Addition or Subtraction Formula to write the expression as a trigonometric function of one number, cos 5degree cos 85degree - sin 5degree sin 85degree Find its exact value.

Solution

cosAcosB-sinAsinB=cos(A+B)

cos5^ocos85^o-sin5^osin85^o

=cos(5+85)^o

=cos90^o

exact value =0