a) Do \(90^o< \alpha< 180^o\) nên \(sin\alpha>0;cos\alpha< 0\). b) Do \(180^o< \alpha< 270^o\) nên \(sin\alpha< 0;cos\alpha< 0\). c) Do \(270^o< \alpha< 360^o\) nên \(sin\alpha< 0;cos\alpha>0\). d) \(\alpha=1280^o=3.360^o+200^o\) \(sin1280^o=sin\left(3.360^o+200^o\right)=sin200^o< 0\). e) \(sin\left(-235^o\right)=sin\left(-235^o+360^o\right)=sin125^o>0\). \(cos\left(-235^o\right)=cos\left(-235^o+360^o\right)=cos125^o< 0\). d) \(sin\left(-1876\right)=sin\left(-1876^o+1800^o\right)=sin\left(-76^o\right)\)\(=-sin76^o< 0\). \(cos\left(-1876^o\right)=cos\left(-76^o\right)=cos76^o>0\).