function Mar19()
m1 = 10; m2 = 20; m3 = 30; n = 5;
A1 = randn(m1,n); A2 = randn(m2,n); A3 = randn(m3,n);
b1 = randn(m1,1); b2 = randn(m2,1); b3 = randn(m3,1);
% Since norm(A1*x-b1)^2 + norm(A2*x-b2)^2 + norm(A3*x-b3)^2 =
%         norm([A1;A2;A3]*x - [b1;b2;b3])^2
x = [A1;A2;A3]\[b1;b2;b3]