2007-12-26 余弦定理 DEV http://www.kwansei.ac.jp/hs/z90010/sugaku1/sankaku/yogen/yogen.htm http://www.ies.co.jp/LoveMath/center/yogen1/yogen1.html http://www004.upp.so-net.ne.jp/s_honma/trigonometry/sinecosine.htm http://www004.upp.so-net.ne.jp/s_honma/urawaza/cosine.htm a*a = b*b + c*c - 2*b*c*cosA