=(cosa·cos15°-sina·sin15°)^2+(sina·cos15°-cosa·sin15°)^2
=cos^2a·cos^2(15°)-2cosa·sina·cos15°·sin15°+sin^2a·sin^2(15°)+sin^2a·cos^2(15°)-2sina·cosa·sin15°·cos15°+cos^2a·sin^2(15°)
=cos^2a·[cos^2(15°)+sin^2(15°)]+sin^2a·[sin^2(15°)+cos^2(15°)]-4cosa·sina·cos15°·sin15°
=cos^2a+sin^2a-sin2a·sin30°
=1-sin2a/2