# MATLAB: Creating a diagonalmatrix with diag and add a value to it

hi,
I'm trying to create a diagonal matrix, with two minor diagonals and one major diagonal. I already created the two minor, but I don't know how to create the main one.
Code:
``%diagonalmatrix D = diag((-1)*k_i,-1) + diag(k_i+k_(i+1)) + diag((-1)*k_i, 1);  ``
diag((-1)*k_i,-1) and diag((-1)*k_i, 1) already work.
How to change diag(k_i+k_(i+1)) ?
Must I create a new matrix with the addition already included for the major diagonal? How to?
Thanks!
I included the concept in the attachment. ``k = sym('k', [1, 8]);A = diag(k(2:end-1),1) + diag(k(2:end-1),-1) + diag(k(1:end-1)+k(2:end));``
``>> AA =[ k1 + k2,      k2,       0,       0,       0,       0,       0][      k2, k2 + k3,      k3,       0,       0,       0,       0][       0,      k3, k3 + k4,      k4,       0,       0,       0][       0,       0,      k4, k4 + k5,      k5,       0,       0][       0,       0,       0,      k5, k5 + k6,      k6,       0][       0,       0,       0,       0,      k6, k6 + k7,      k7][       0,       0,       0,       0,       0,      k7, k7 + k8]``