﹣¼π＜α﹤¾π sin（¼﹣α）＝√5／5 sinα＝

﹣¼π＜α﹤¾π sin（¼﹣α）＝√5／5 sinα＝
﹣¼π＜α﹤¾π sin（¼﹣α）＝√5／5 sinα＝

﹣¼π＜α﹤¾π sin（¼﹣α）＝√5／5 sinα＝
¼π＜α﹤¾π
sin（¼π﹣α）＝√5／5
是不是少了一个π
-¼π＜α﹤¾π
-π/2<π/4-a<π/2
cos(π/4-a)>0
cos(π/4-a)=2√5/5
sina=sin[π/4-(π/4-a)]
=sinπ/4cos(π/4-a)-sin(π/4-a)cosπ/4
=(√2/2)[2√5/5-√5/5]
=√10/10