三维向量点乘
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Vector3D a(x_1,y_1,z_1),b(x_1,y_1,z_1);
a · b = x_1 x_2 + y_1 y_2 + z_1 z_2;
a · b = |a| |b| Cosθ = √(x_1^2 y_1^2 z_1^2) √(x_2^2 y_2^2 z_2^2) Cosθ;
θ = arcCos(ab);
a·b > 0 向量夹角为锐角;
a·b = 0 向量夹角为直角;
a·b < 0 向量夹角为钝角;
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