question_answer

Which of the following is incorrect?

A) \[{{\cos }^{4}}\theta -{{\sin }^{4}}\theta ={{\cos }^{2}}\theta -{{\sin }^{2}}\theta \]

B) \[1+{{\tan }^{2}}\theta ={{\operatorname{csec}}^{2}}\theta \]

C) \[\sin 40{}^\circ +\cos 50{}^\circ =2\sin 40{}^\circ \]

D) \[{{\sin }^{2}}\theta +{{\cos }^{2}}(-\theta )=-1\]