Change in Tandem Practice Test

Based on engineering dynamics, stress is $\sigma(t)=\dfrac{40t+20}{t+1}$ and strain is $\varepsilon(t)=\dfrac{1}{200},\sigma(t)$ for $t\ge0$. What happens to strain if stress increases as $t$ increases?

In the context of economic growth, investment is $I(t)=0.3t^2+2t+50$ and GDP is $G(t)=0.05t^3+0.6t^2+3t+200$ for years $t\ge0$. How does a change in investment affect GDP as time increases?