% check exp(-x) + x
x=0:4;

for j=1:(length(x)-1)
    a=x(j);
    b=x(j+1);
    xx=linspace(a,b,100);
    figure(1);hold on;
    plot(xx,xx,xx,exp(-xx)+xx);
    fill([a b b a a],[a a b b a],[0.8 0.8 0.8],'FaceAlpha',0.2,'EdgeAlph',0.1);
end