Question

已知sin18°=(√5-1)/4,则(cos36°)^2+(sin18°)^2=

Answer 1

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.