题目
已知sin18°=(√5-1)/4,则(cos36°)^2+(sin18°)^2=
提问时间：2021-03-25
答案
因为 sin18°=(√5-1) /4,
所以 (cos 36°)^2 +(sin 18°)^2 =(1 +cos 72°)/2 +(sin 18°)^2
=(1 +sin 18°)/2 +(sin 18°)^2
=...(代入)
=3/4.
= = = = = = = = =
半角公式：
(cos x/2)^2 =(1 +cos x)/2.
记不住的话,用二倍角公式来推.
所以 (cos 36°)^2 +(sin 18°)^2 =(1 +cos 72°)/2 +(sin 18°)^2
=(1 +sin 18°)/2 +(sin 18°)^2
=...(代入)
=3/4.
= = = = = = = = =
半角公式：
(cos x/2)^2 =(1 +cos x)/2.
记不住的话,用二倍角公式来推.
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