Question #babaa

1 Answer
Sep 24, 2014

Let #f(t)=vec{r}(t)cdot(vec{r}'(t)\times vec{r}''(t))#.

#f(t)=(-4t^{-1}-2tlnt+2lnt+t+1)e^t#

#f'(t)=(4t^{-2}-2t^{-1}+t-2tlnt)e^t#

Let us look at some details.

By differentiating component by component with respect to #t#,

#vec{r}(t)=(e^t,-lnt,t^2)#
#vec{r}'(t)=(e^t,-t^{-1},2t)#
#vec{r}''(t)=(e^t,t^{-2},2)#

#vec{r}'(x)\times r''(t) =|{:(vec{i}, vec{j},vec{k}),(e^t,-t^{-1},2t),(e^t,t^{-2},2):}|#

#=vec{i}|{:(-t^{-1},2t),(t^{-2},2):}|-vec{j}|{:(e^t,2t),(e^t,2):}|+\vec{k}|{:(e^t,-t^{-1}),(e^t,t^{-2}):}|#

#=(-4t^{-1},2te^t-2e^t,(t+1)t^{-2} e^t)#

Let #f(t)=vec{r}(t)cdot(vec{r}'(t)\times vec{r}''(t))#

#=(e^t,-lnt,t^2)cdot(-4t^{-1},2te^t-2e^t,(t+1)t^{-2} e^t)#

#=-4t^{-1} e^t-2te^tlnt+2e^t lnt+(t+1)e^t#

#=(-4t^{-1}-2tlnt+2lnt+t+1)e^t#

By Product Rule,

#f'(t)=(4t^{-2}-2lnt-2+2t^{-1}+1)e^t+(-4t^{-1}-2tlnt+2lnt+t+1)e^t#

#=(4t^{-2}-2t^{-1}+t-2tlnt)e^t#