Respuesta :

[tex]\bf f(t)=10t^2+5t+1\implies \stackrel{\textit{slope of tangent}}{\cfrac{df}{dt}}=20t+5\implies 1=20t+5 \\\\\\ -4=20t\implies -\cfrac{4}{20}=t\implies -\cfrac{1}{5}=t[/tex]