Some syntax errors there in your code. Try this
v = -50:10:50
% Initialize x for outside the range.
x = 0.01 * ones(size(v));
indexesInRange = (v >= 49.9) & (v <= 50.1) % Only the last element, 50, will be in range.
% Assign a formula to x at those places where v is in range.
x(indexesInRange) = (.01 * (v(indexesInRange)-50)) ./ (exp(0.1 * (v(indexesInRange)-50)) - 1)