vẫn chỉ là toán lượng giác 10

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duchieu300699

minhhieu2468 said:
cos^2 x+cos^2(x+2II/3)+cos^2(x+4II/3)!
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$cos^2x+cos^2(x+\dfrac{2\pi }{3})+cos^2(x+\dfrac{4\pi }{3})$

= $cos^2x+(cosx.cos\dfrac{2\pi }{3}-sinx.sin\dfrac{2\pi }{3})^2+(cosx.cos\dfrac{4\pi }{3}-sinx.sin\dfrac{4\pi }{3})^2$

= $cos^2x+(\dfrac{-cosx-sinx\sqrt{3}}{2})^2+(\dfrac{-cosx+sinx\sqrt{3}}{2})^2$

= $cos^2x+\dfrac{cos^2x+2\sqrt{3}cosx.sinx+3sin^2x}{4}+\dfrac{cos^2x-2\sqrt{3}cosx.sinx+3sin^2x}{4}$

= $\dfrac{3cos^2x+3sin^2x}{2}=1,5$

 
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