[tex]a) \ tg37^{o}\cdot sin53^{o} = \frac{sin37^{o}}{cos37^{o}}\cdot sin(90^{o}-37^{o}) = \frac{sin37^{o}}{cos37^{o}}\cdot cos37^{o} = sin37^{o}\\\\b) \ \frac{cos18^{o}}{tg72^{o}} = \frac{cos(90^{o}-72^{o})}{\frac{sin72^{o}}{cos72^{o}}} = \frac{sin72^{o}}{\frac{sin72^{o}}{cos72^{o}}} = sin72^{o}\cdot\frac{cos72^{o}}{sin72^{o}}} = cos72^{o}[/tex]
