function Feb27()
% norm(Ax-b)^2 = (a11*x1 + a12*x2 - b1)^2 + 
%                (a21*x1 + a22*x2 - b2)^2 + 
%                (a31*x1 + a32*x2 - b3)^2
% partial with respect to x1 =
%                2*a11*(a11*x1 + a12*x2 - b1)^2 + 
%                2*a21*(a21*x1 + a22*x2 - b2)^2 + 
%                2*a31*(a31*x1 + a32*x2 - b3)^2
% Equating to zero gives
%    A(:,1)'A(:,1)x1 + A(:,1)'A(:2)x2 = A(:,1)'b         (1)
%
% partial with respect to x2 =
%                2*a12*(a11*x1 + a12*x2 - b1)^2 + 
%                2*a22*(a21*x1 + a22*x2 - b2)^2 + 
%                2*a32*(a31*x1 + a32*x2 - b3)^2
% Equating to zero gives
%    A(:,2)'A(:,1)x1 + A(:,2)'A(:2)x2 = A(:,2)'b        (2)
%
% Assemble (1) and (2) into matrix-vector form...
%
%          A(:,1)'A(:,1)   A(:,2)'A(:,1)    x1       A(:,1)'b1
%                                               =
%          A(:,2)'*A(:,1)  A(:,2)'A(:,2)    x2       A(:,2)'b2

% i.e., A'Ax = A'b
%
