Linear Regression: Spectrophotometric determination of Mn

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In the spectrophotometric determination of manganese (as permanganate) a linear dependence of the absorbance for a given concentration range is obtanied:

c/(mmol/L)0.1860.4590.7280.9061.0941.3511.6471.838
A0.0920.202 0.3210.401 0.4980.6020.7310.820
The formula for this linear dependence can be written as y=b*x+a, where y corresponds to the absorbance and x to the concentration (or A=b*c+a). The parameters b and a, are the slope of the line and the intercept with the ordinate, respectively.
In MathCAD, the data series are defined as vectors (one-column matrices). The syntax for the slope and the intercept, as well as the correlation of the two sets of data is simple, as illustrated in the following example:
The unknown concentrations of samples can be determined from

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