Matlab - Como hacer una variable simbolica

function Interact2(Pos)

global P0 P1 H1 H2 x y

% This part is executed when you run it for the first time.

% In that case, the number of input arguments (nargin) == 0.

if nargin == 0

cont=1;

salir=0;

while salir==0

    [p1 p2 b]=ginput(1);

    if isempty(b);

        salir=1;

    else

        x(cont,1)=p1;

        y(cont,1)=p2;

        plot(x(cont,1),y(cont,1),'x')

        hold on

        axis([0 1000 0 1000])

        cont=cont+1;

    end

end

    % I do not know how to do this without global variables?

    % GCA = Get handle for Current Axis

    P0 = impoint(gca,x(1),y(1));

    setString(P0,'P start');

    P1 = impoint(gca,x(2),y(2));

%     setPositionConstraintFcn(P1,@(pos) [-pos(2)*sind(10) pos(2)])

    setString(P1,'P slutt');

    % Call subfunction

    DrawLagrange(P0,P1);

    % Add callback to each point

    addNewPositionCallback(P0,@Interact2);

    addNewPositionCallback(P1,@Interact2);

%addNewPositionCallback(PB,@Interact);

else

    % If there _is_ some input argument, it has to be the updated

    % position of a moved point.

    % Display X and Y coordinates of moved point

%     Pos

    % Important: remove old plots! Otherwise the graph will get messy.

    delete(H1)

    delete(H2)

	DrawLagrange(P0,P1)

end

end

function DrawLagrange(P0,P1)

global H1 H2 P X Y Px Py

P = zeros(1,2);

% Get X and Y coordinates for the 3 points.

P(1,:)  =   getPosition(P0);

P(2,:)  =   getPosition(P1);

H1 = plot(P(:,1), P(:,2), 'ko--', 'MarkerSize', 12);

hold on

[func,bez]=bezie(P);

CurveX=bez(:,1);

CurveY=bez(:,2);

x=P(:,1);

y=P(:,2);

n=length(x)-1;

syms Ctx;  % Este es el problema

syms Cty;

syms t;

Ctx=0;

Cty=0;

for i=0:n

    comb = combinacion(i,n);

    Ctx = Ctx + (((x(i+1,1)).*(comb).*(t.^i)).* ((1-t).^(n-i)))

    Cty = Cty + (((y(i+1,1)).*(comb).*(t.^i)).* ((1-t).^(n-i)))

end

t=[0:0.1:1];

X1=Ctx

Y1=Cty

X=char(X1)

Y=char(Y1)

Px=eval(X1)

Py=1000-eval(Y1)

H2 = plot(CurveX, CurveY,'LineWidth',4);

end