To calculate the residual of the data point (1, 16.88) using the line of best fit \( y = -0.9x + 18 \), we first need to find the predicted value of \( y \) when \( x = 1 \).
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Substitute \( x = 1 \) into the equation of the line: \[ y = -0.9(1) + 18 \] \[ y = -0.9 + 18 = 17.1 \]
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The predicted value of \( y \) when \( x = 1 \) is \( 17.1 \).
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The residual is calculated as the actual value minus the predicted value: \[ \text{Residual} = \text{Actual Value} - \text{Predicted Value} \] \[ \text{Residual} = 16.88 - 17.1 \] \[ \text{Residual} = -0.22 \]
So, the residual of the data point (1, 16.88) is \(\boxed{-0.22}\).