$[$Recursion Tree$]$: The second approach for solving a recurrence is the recursion tree method. Each node represents the cost of a single subproblem. You sum the costs within each level of the tree to

obtain the per$-$level costs, and then you sum all the per$-$level costs to determine the total cost of all levels of the recursion. This method is best for building intuition for a good guess.

Using the recursion tree method, guess a good asymptotic upper bound on the solution of the following recurrence.

$($a$)$

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T$($n$)=2$ T$($n $/5)+$n$^\{3\}.$

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