Question 747302
in the equation

{{{
cos(A) cos(-A)- sin(A)sin(-A)=1
}}}

we notice the evenness and oddness of cosine and sine:

{{{
cos(-theta)=cos(theta)
}}}

and

{{{
sin(-theta)=-sin(theta)
}}}

therefore...

{{{
cos(A) cos(-A)- sin(A)sin(-A)=cos^2(A)+sin^2(A)=1
}}}


:)