tan(\alpha +\beta )=\frac{sin(\alpha )cos(\beta )+cos(\alpha )sin(\beta )}{cos(\alpha )cos(\beta )-sin(\alpha )sin(\beta )}

上下都除以

cos(\alpha )cos(\beta )

\frac{\frac{sin(\alpha )cos(\beta )+cos(\alpha )sin(\beta )}{cos(\alpha )cos(\beta )}}{\frac{cos(\alpha )cos(\beta )-sin(\alpha )sin(\beta )}{cos(\alpha )cos(\beta )}}= \frac{tan(\alpha )+tan(\beta )}{1-tan(\alpha)tan(\beta ) }

符号的相反性,是由cos(\alpha +\beta )带来的

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