已知向量点乘a.b向量叉乘axb求向量a和向量b的夹角因为：a.b=|a||b|cos|axb|=|a||b|sin所以：tan = |axb|/(a.b)所以：当cos大于0, 也就是a.b>0 时, = arctan[|axb|/(a,b)]而当a.b<0时, = 180° + arctan[|axb|/(a,b)]
假设a×b=A,a·b=Ba×b=absinΘa·b=abcosΘ两式相除 A/B =sinΘ/cosΘ=tanΘ
