The question is-
The following data shows atmospheric pollutants $y_{i}$ (relative to an EPA standard) at half hourintervals $t_{i}$ $$ \begin{array}{|c|c|} \hline t_{i} & 1 & 1.5 & 2 & 2.5 & 3 & 3.5 & 4 & 4.5 & 5 \\ \hline y_{i} & -0.15 & 0.24 & 0.68 & 1.04 & 1.21 & 1.15 & 0.86 & 0.41 & -0.08\\ \hline \end{array} $$
What form of the curve should I assume to solve this equation, quadratic or linear? The solution given in my college's module assumes the curve as $y=a_{2}t^{2}+a_{1}t+a_{0}$ but won't explain why.
Or could this question be incomplete? It seems to me