  For three-body problem, R.Montgomery [3] proved a reconstruction formula
which calculates the overall rotation relating two similar triangle
configurations if the initial triangular configuration is similar to the
configuration formed at some later time. In this paper, we extend the formula
so that it gives the angle of rotation for a single particle without requiring
the similarity of initial and final configurations. The proof is different from
that in [3] and uses fundamental calculus. Moreover, we answered a question
proposed in [3].
