tentukan nilai dari cos 67,5

cos 2A = 2cos^2 A -1
2cos^2 A = cos 2A +1
cos^2 A = (cos 2A +1)/2
cos A = √{cos 2A +1)/2}
cos 67,5° = √{cos 2×67,5+1)/2}
= √{ cos 135+ 1)/2}
= √(-1/2√2+1)/2
= √(-1/4√2+1/2)
= √(1/2-1/4√2)