Cos⁴15°-sin⁴15°adalah

cos⁴ 15° + sin⁴ 15° 

= ( cos² 15° + sin² 15° )² - 2 cos² 15° sin² 15° 

= ( 1 )² - (1/2) ( 4 sin² 15° cos² 15° ) 

= 1 - (1/2) ( 2 sin 15° cos 15° )² 

= 1 - (1/2) [ sin ( 2(15°)) ]² 

= 1 - (1/2) ( sin 30° )² 

= 1 - (1/2) (1/2)² 

= 1 - (1/8) 

= 7/8