=cos(15°-x)-√3/2cos(15°+x)-sin30°sin(15+x)
=cos(15°-x)-[cos30°cos(15°+x)+sin30°sin(15+x)]
=cos(15°-x)-cos[30°-(15°+x)]
==cos(15°-x)-cos(15-x)
=cos(15°-x)-√3/2cos(15°+x)-sin30°sin(15+x)
=cos(15°-x)-[cos30°cos(15°+x)+sin30°sin(15+x)]
=cos(15°-x)-cos[30°-(15°+x)]
==cos(15°-x)-cos(15-x)