Problem $6(8$ Points$)$ Evaluate: $($a$)($t $2)$$($t $4),($b$)$ $($t $($$/2))$ sin$($t $+($$/2)),($c$)$ R $$ e $2$t $(3$t$)$dt$,($d$)$ dx$/$dt$,$ where x$($t$)=$ t $2$ sgn t $+$ t$[$u$($t$)$ u$($t $$ a$)],$ For the first two use the property: x$($t$)$$($t $$ t$0)=$ x$($t$0)$$($t $$ t$0)$