How to Plot Truncated Summation Functions without any add-ons

Hello, I'm trying to figure out how to go about plotting a truncated summation function such as

without the use of any add-ons. Almost every resourse I'm finding for Matlab requires a toolbox add-on that I do not have access to, such as PDF from the stats/machine toolbox, or syms from the symbolic math toolbox. If anyone could give a hint on how to approach this, I would apprectate it.

Answer 1

[David Hill]

y=@(M,x)2/pi*sum((-1).^((1:M)-1).*cos(2*(1:M)-1)*pi/2.*x'./(2*(1:M)-1),2)+0.5;
x=-10:10;
plot(x,y(3,x),x,y(10,x),x,y(20,x));%plot vs. different values of M

[Anthony Koning]

I appreciate the answer, but the graph of this plot doesn't make much sense to me, just being straight lines. I understand it compares a handful of values for N, but shouldn't the graph have a sinusoidal shape since the graph is a sum of cosines?