Your ‘str’ variable must come before your annotation call.
You are missing curly braces in the ‘\infty’ argument.
Change the order of the statements, add the curly braces, and it works:
str={'(1) $$y= \frac{1}{(1+x)^2} = \sum_{k=1}^\infty{k(-x)}^{k-1}$$',...
'(2) $$y= sin^2(x) = \sum_{k=1}^\infty{(-1)}^{k+1}\frac{2^{2k-1}x^{2k}}{(2k)!}$$'...
'(3) $$y=e^{-x^2} = \sum_{k=1}^\infty{(-1)}^k\frac{x^{2k}}{k!}$$'};
annotation('textbox','interpreter','latex','String',str,'FitBoxToText','on');
