a) \( \cos ^{2} x-3 \operatorname{sen}^{2} x=0 \)

\( \cos ^ { r } x - \sin x = \frac { 1 } { r } \)

\( \left. \begin{array} { l } { \frac { \operatorname { Arctan } ( \tan ( 5 ) ) } { \operatorname { Sen } ( \operatorname { arcsen } ( 5 ) ) } + \operatorname { Tan } \{ \operatorname { Cos } ( \operatorname { Cos } ^ { - 1 } ( x ) ) \} = - 0.7 } \\ { 16 ^ { \circ } } \\ { - 12 ^ { \circ } } \\ { - 8 ^ { \circ } } \\ { 4 ^ { \circ } } \end{array} \right. \)

\( A B C \) is a triangle. \( A C K K \nabla .9 \mathrm{~cm} \) Angle \( B=90^{\circ} \) Angle \( C=38^{\circ} \) Calculate the length of \( B C \). Give your answer correct to 3 significant figures

\( | ( \sin m ) ( \sin \frac { 1 } { \mu ^ { 2 } } ) | \leq \sin \frac { 1 } { m ^ { 2 } } \)

\( A B C \) is a triangle. \( A C K K .9 \mathrm{~cm} \) Angle \( B=90^{\circ} \) Angle \( C=38^{\circ} \) Calculate the length of \( B C \). Give your answer correct to 3 significant figures.