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Question $1$

$5pt$

Which $of$ the following $is$ the best substitution for the indefinite integral

${\displaystyle \underset{\phantom{}}{\overset{\phantom{}}{}}}\frac{x^3}{\sqrt[\phantom{2}]{x^2-1}}dx$

$x=sin$

$x=sec$

$u=x^2-1$

$x=tan$