Conversion to function_handle from double is not possible. in matlab

في الماتلاب هنا يظهرلي خطا كيفي اصححه حيث اني اريد ان احصل عل exact solution واقارنه بطريقه euler بالرسم

clear all
f=@(x) (-2*x.^3+12*x.^2-20*x+8.5);
exact_sol= @(x)(-0.5*x.^4+4*x.^3-10*x.^2+8.5*x);
a=0; b=4; h=0.5;
y0=1;
n=(b-a)/h;
f0=f(a);
x=[]; f=[];y=[];
y(1)=y0+f0*h;
x=0:h:4;  
display('x                    y                  y(i+1)')
display('================================================')
fprintf('%5f    %16f            %10f\n',[a,;y0;y(1)])

for i=1:n
    x(i)=a+i*h;
    f(i)=-2*x(i)^3+12*x(i)^2-20*x(i)+8.5;
    exact_sol(i)= -0.5*x(i)^4+4*x(i)^3-10*x(i)^2+8.5*x(i)
    y(i+1)=y(i)+f(i)*h;
fprintf('%5f    %16f           %10f\n',[x(i);y(i);y(i+1)])
end
plot(x,y)                                     %plot Euler
hold on     
plot(x,exact_sol)% plots the exact solution to this differential equation
legend('Euler','Exact');           % adds a legend
grid on

EULER.m