A model for the food-price index (the price of a representative "basket" of foods) between 1984 and 1994 is given by the function


Where t is measured in years since midyear 1984, so [math], and I(t) is measured in 1987 dollars and scaled such that I(3)=100. Estimate to two decimal places the times when food was cheapest and most expensive during the 1984-1994 period.

cheapest at t =

most expensive at t =