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In computational 3D geometric problems involving rotations, it is often that people have to convert back and forth between a rotational matrix and a rotation described by an axis and a corresponding angle. For this purpose, Rodrigues' rotation formula is a very popular expression to use because of its simplicity and efficiency. Nevertheless, while converting a rotation matrix to an axis of rotation and the rotation angle, there exists ambiguity. Further judgement or even manual interference may be necessary in some situations. An extension of the Rodrigues' formula helps to find the sine and cosine values of the rotation angle with respect to a given rotation axis is found and this simple extension may help to accelerate many applications., Comment: 9 pages, 1 figure, minor typo corrected