as it allows one to adjust the estimate "x" by some amount "c" and measure the square of the adjustment in terms of the original estimate and its square. Furthermore, ("x" + "c")2 ≈ "x"2 + 2"xc" when "c" is close to 0, because the tangent line to the graph of "x"2 + 2"xc" + "c"2 at "c" = 0, as a function of "c" alone, is "y" = 2"xc" + "x"2. Thus, small adjustments to "x" can be planned out by setting 2"xc" to "a", or "c" = "a"/(2"x").