The Point That Flattens the Line
At $x = 1, 2, 3, 4, 5$ the observed values are $y = 9, 4, 3, 5$ and an unknown fifth value $y_5$. For which $y_5$ is the least-squares slope of $y$ on $x$ exactly zero? Give a reduced fraction or decimal.
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Source: Original exercise written for AxiomIQ.