二上1-4和角與差角公式(如何算出sin 20．sin 40．sin 80？)

參考資料出處：
 １.
http://math1.ck.tp.edu.tw/%E9%99%B3%E5%98%AF%E8%99%8E/%E5%B0%8F%E8%99%8E/99%E8%AA%B2%E7%B6%B1/%E7%AC%AC%E4%B8%89%E5%86%8A/%E7%B7%B4%E7%BF%92%E9%A1%8C/99%E8%AA%B2%E7%B6%B13-1-4%E4%B8%89%E8%A7%92-%E5%B7%AE%E8%A7%92%E5%85%AC%E5%BC%8F(%E7%B7%B4%E7%BF%92%E9%A1%8C)%E8%AD%89%E6%98%8E%E9%A1%8C.pdf
2.
http://tw.knowledge.yahoo.com/question/question?qid=1406081002987