A) \[{{\cos }^{4}}\theta -{{\sin }^{4}}\theta ={{\cos }^{2}}\theta -{{\sin }^{2}}\theta \]
B) \[1{{\tan }^{2}}\theta ={{\sec }^{2}}\theta \]
C) \[\sin 40{}^\circ +\cos 50{}^\circ =2\sin 40{}^\circ \]
D) \[{{\sin }^{2}}\theta +{{\cos }^{2}}\theta =2\]
Correct Answer: D
Solution :
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