Determine os valores de: a)

Determine os valores de: a) $y=3 \cos 540^{\circ}-2 \operatorname{sen} 90^{\circ}+\operatorname{tg} 180^{\circ}$ b) $y=4 \operatorname{sen} 900^{\circ}-2 \cos 630^{\circ}+\operatorname{sen} 720^{\circ}$