https://math.stackexchange.com/questions/550991/if-fx-cos-x-explain-without-taking-the-derivative-how-you-would-find-t/550993
The cycle occurs, not three times but 4 times. Thus every 4 derivatives taken you return to lớn cos(x). Therefore, since 99=96+3, you need the 3rd derivative of cos(x) which is sin(x).

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Are you familiar with the hàng hóa rule? f(x)=u(x)v(x)\ f'(x)=u'(x)v(x) + u(x)v'(x) In your case u(x)=cos(x), v(x)=sin(x). EDIT Sorry again for the typos, Here's what you should do. The ...
https://math.stackexchange.com/questions/1319003/why-this-this-approach-to-solving-a-trig-equation-yield-invalid-solutions-alongs
First you multiplied by sin4x. When this is zero, this makes the equation satisfied automatically. That's 8 extra roots... Also, because you squared, you lost the information that sin4x+cos4x = +1: ...
https://math.stackexchange.com/questions/2677140/show-that-j-is-diagonalizable-and-find-an-eigenbasis
I don't know what you've learned so far, but I'm guessing that you know how to khuyến mãi with ordinary matrices. Given that 1, cos x, & sin x size a basis of V, you can represent any element a+bcos x+csin x ...

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https://math.stackexchange.com/questions/3066804/is-it-legal-to-divide-an-equation-by-a-trigonometric-expression-which-may-equal
Yes you can, because cos(4x)=0 cannot correspond to a solution (this would imply sin(4x)=pm1 e0 ). The usual way lớn handle such situations is khổng lồ perform a case study: cos(4x)=0 o ...
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