[b] ABC is a right-angled triangle at \( C, A C=6 \mathrm{~cm} ., \mathrm{BC}=8 \), Find: 1 ) \( \cos A \cos B-\sin A \sin B \) (a) \( m(\angle B) \)

15. Simplifa cada erpresion a) \( \operatorname{sen} \alpha \cdot \cot \alpha \) b) \( \tan \alpha+\cot \alpha \) c) \( \operatorname{sen} \alpha \cdot(\tan \alpha+\cot \alpha) \) 16. Demuestra las siguientes identidades

Ao transformar o arco de medida \( 12^{\circ} \) em radianos e o arco de medida de \( \frac{\pi}{9} \) radianos em graus, obtemos os arcos, respectivamente: O a. \( \frac{\pi}{15} \) radianos \( \underline{\underline{\text { e } ~} 20^{\circ}} \) o b. \( \frac{\pi}{30} \) radianos \( \underline{\underline{\text { e } 30^{\circ}}} \) O c. \( \frac{\pi}{15} \) radianos \( \underline{\underline{\text { e } 40^{\circ}}} \) O d. \( \frac{\pi}{12} \) radianos \( \underline{\underline{\text { e } 20^{\circ}}} \) O e. \( \frac{\pi}{12} \) radianos \( \underline{\underline{\text { e } 40^{\circ}}} \)

[b] ABC is a right-angled triangle at \( C, A C=6 \mathrm{~cm} ., \mathrm{BC}=8 \), Find: 1 ) \( \cos A \cos B-\sin A \sin B \) (a) \( m(\angle B) \)

\( \frac { 1 } { 1 + \tan 1 ^ { \circ } } + \frac { 1 } { 1 + \tan 2 ^ { \circ } } + \frac { 1 } { 1 + \tan 3 ^ { \circ } } + \cdots + \frac { 1 } { 1 + \tan 89 ^ { \circ } } = ? \)

15. Simplifica cada expresión. a) \( \operatorname{sen} \alpha \cdot \cot \alpha \) b) \( \tan \alpha+\cot \alpha \) c) \( \operatorname{sen} \alpha \cdot(\tan \alpha+\cot \alpha) \)