Hello

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Using rough expanding :

x^3 coefficients :

x² coefficients :

x coefficients :

constant coefficients :

You're right, you can easily get B.

Substituting in the previous equations :

From the first equation, we have and from the fourth,

Substituting again :

Therefore and

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There is a faster way, in noticing that

But if you didn't study the complex numbers, it won't be easy for you to grasp it immediately

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There is another way, involving differentiation (you do the same trick as you did for B, after you have differentiated )