Question 747302
Using symmetry of the functions, {{{cos(A)=cos(-A)}}}, and {{{sin(-A)=-sin(A)}}}.  Check on the Unit Circle to be sure this makes sense.


Those facts let us say this step.
{{{cos(A)cos(-A)-sin(A)sin(-A)=cos(A)cos(A)-sin(A)*(-1)sin(A)}}}
={{{cos(A)cos(A)-(-1)sin(A)*sin(A)}}}
={{{(cos(A))^2+(sin(A))^2=1}}}
DONE.