You need the symbolic toolbox, and you use the symsum() function
Note, however, that symsum has the limitation that you cannot use symbols to index an array. For example you cannot say
A = [2 5 8];
syms k
symsum( k*A(k).^k, k, 1, 3)
If you need to index into an array as part of the formula, then MATLAB requires you to form the definite expressions and then sum() them:
K = 1 : 3;
sum( K .* A(K) .^ K )
Notice the array is not being indexed by a symbol here.
