Note:  the first two questions are independent from the remaining 
problems.  Please attempt 3-4 even if you can't get 1-2 to work.

1.  First evaluate (i) at x_j, then approximate y''(x_j) using 5.91 in 
terms of y(x_{j-1}), y(x_j), and y(x_{j+1}).  Finally, denote y(x_j) as 
y_j.

2.  Might help to do this backwards.  Write the matrix and vector for the 
case where n=4.  Multiply out A*y and convert to a system of equations.  
Make sure that the first and last equations match with (ii.) and (iii).  
The middle equations should match with (1) for j=1,...,n-1.

3.  Use GEpivot as a function, you should not have to code A into the 
script.  You may check your answer with yi=A\b; (which uses Matlab's 
built-in GE).

4.  Don't forget to time GEpivot as well!

5.  [1;2*ones(n-1,1);1] gives a vector of n-1 2's in the middle, with a 1 
on top and bottom.