function pexps(xi,a,b,ax)
% PEXPS  plots the two exponential functions
xx = [0:0.1:xi];
plot(xx, a(1) + a(2)*exp(a(3)*xx),'r')
axis(ax)
xx = [xi:0.1:240];
plot(xx,b(1) + b(2)*exp(b(3)*xx),'g')
axis(ax)

