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Question Number 31745 by pieroo last updated on 13/Mar/18

$$\mathrm{pls},\mathrm{why}\mathrm{is}{\cos }^4-{\sin }^4={\cos }^2-{\sin }^2?$$

Answered by Tinkutara last updated on 13/Mar/18

$${\cos }^4\theta -{\sin }^4\theta$$$$=({\cos }^2\theta -{\sin }^2\theta )({\cos }^2\theta +{\sin }^2\theta )$$$$=({\cos }^2\theta -{\sin }^2\theta )\left(1\right)$$$$={\cos }^2\theta -{\sin }^2\theta$$

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