i)
Since
,
You can use long division and then divide by
(since
) to obtain the desired result. It's rather unintuitive, though.
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ii) Consider the expansions of
.
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iii) There are six solutions to the equation in part i). By performing the substitution in ii), you've reduced it down to three, by making three of those solutions the same as the other three.
Find these three unique solutions and use the product of roots and the properties of
in the first and second quadrants to obtain the answer.