记18°=t
å5t=90°
sin3t=cos2t
sin(t+2t)=cos2t
sintcos2t+costsin2t=cos2t
sint(1-2sin²t)+2sint(1-sin²t)=1-2sin²t
è®°sint=x,å
x(1-2x²)+2x(1-x²)=1-2x²
(x-1)(4x²+2x-1)=0
xâ 1,æ以4x²+2x-1=0,åæ£æ ¹å¾ï¼
x=(â5-1)/4å³ä¸ºsin18°çå¼.
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