$(1$ point$)($Exercise $1\mathrm{.}26)$ a$)$ Express $d^{(6)}$ as a function of $x=i^{(3)}($use $x$ as the variable$)$

$d^{(6)}(x)=$

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b$)$ Express $i^{(6)}$ as a function of $x=d^{(4)}($use $x$ as the variable$)$

$i^{(6)}(x)=$

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