Symbol x_N

The passage around this formula

The GUM’s central move is to treat every measured or estimated quantity a result depends on as a random variable with its own estimated standard uncertainty, and then to propagate those uncertainties through the mathematical relationship that produces the final result. If a measurement result y is a function of N input quantities, y = f($x_1$, $x_2$, $\ldots$, $x_N$) , the combined standard uncertainty is obtained from a first-order Taylor expansion of that function:

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