Hello

Let :

Because you're given the formula above, divide by (because it's different to 0, we can do it)

Therefore :

Now, it would be great if it could be simplified.

You can notice that it looks like a lot de Moivre's formula :

So we're gonna make a transform... According to the unit circle (or to your lesson) :

Great !

Therefore

Now we can apply de Moivre's formula :

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Conclude