this is my working out: http://i.imgur.com/1hsQS.jpg i sort of figured out how to do this a few mins ago lol. it doesnt seem too hard. it's sort of like... multiplying the first number in the matrix A by it's position in the matrix (x1 * x1) which is basically the coordinates of the value, then doing that for all numbers in the matrix A. am i right? have i fully answered the question? edit: in my working out, the xTAx = sigma sigma axx was taken from my course manual.