An analyst examines the effect that various variables have on crop yield.

He estimates the following model:y = ?₀+ ?₁x₁+ ?₂x₂+ ?₃x₃+ ?

Whereyis the average yield in bushels per acre,x₁is the amount of summer rainfall,x₂is the average daily use in machine hours of tractors on the farm, andx₃is the amount of fertilizer used per acre. A portion of the regression results is shown in the accompanying table.

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