clc;
a0=[1 10 0.2 1];
options=optimset('lsqnonlin');
options.TolX=0.01;options.Display='off';options = optimset('Jacobian','on');
x=[0:0.2:4]';
y=3*exp(-0.4*x)+12*exp(-3.2*x);
a=lsqnonlin(@twos,a0,[],[],options,x,y);