Answer:
see the procedure
Step-by-step explanation:
we know that
[tex]cos(X - Y) = cosX cosY + sinX sinY\\cos(X + Y) = cosX cosY - sinX sinY[/tex]
so
[tex]cos(X - Y)-cos(X + Y)=cosX cosY + sinX sinY-(cosX cosY - sinX sinY)[/tex]
[tex]cos(X - Y)-cos(X + Y)=cosX cosY + sinX sinY-cosX cosY + sinX sinY[/tex]
[tex]cos(X - Y)-cos(X + Y)=sinX sinY+ sinX sinY[/tex]
[tex]cos(X - Y)-cos(X + Y)=2sinX sinY[/tex] ----> is verified
