$(10$ points$).$ tight$($upper bound$)($lower bound$).T(1)=(1).($a$)T(n)=4T(\frac{n}{2})+O(n)($b$)T(n)=3T(\frac{n}{\sqrt[\phantom{2}]{2}})+O(n^4)($c$)T(n)=2T(\frac{n}{2})+5nlogn($d$)T(n)=T(n-2)+(n)$