原式=(cosacos15-sinasin15)+(cosacos15+sinasin15)-√3/2*(2cos²a-1)
=2cosacos15-√3cos²a+√3/2
=2cosa(cos60-45)-√3cos²a+√3/2
=2cosa(cos60cos45+sin60sin45)-√3cos²a+√3/2
=[(√6+√2)/2]cosa-√3cos²a+√3/2
=[(√6+√2-2√3)/2]cosa+√3/2