1 - sin^15 / 2 cos^pi/8 -1

= 7/2 * sgrt2 *(2sin15pi/8cos15pi/8)= 7/2 *sgrt2*sin15/4=7/2*sgrt2*sin(-pi/4)= =7/2*sgrt2*(-sgrt2/2)= -7/2= - 3,5