Find ddt$[$r$($t$)$ u$($t$)]$ and ddt$[$r$($t$)$ u$($t$)]$ in two different ways.r$($t$)=$ cos$($t$)$i $+$ sin$($t$)$j $+$ tk$,$ u$($t$)=$ j $+$ tk$($a$)$ddt$[$r$($t$)$ u$($t$)]$