# DAE in Matlab: ode15s

I have to solve this problem

$$\left\{\begin{matrix} y''=4y'+\frac{1}{h}-7y\\ y(1)=1\\ y'(1)=1 \end{matrix}\right. \text{where} \; h=5y'-2y$$

as it were a DAE (I know I could just substitute $h$ into the equation, but this is just an example, because in reality the problem I have to solve is a DAE and more complex than this). When I use ode45 and treat the problem as a second order differential equation, the graph $t$ Vs. $y$ is but when I treat it as a DAE, the graph is completely different and I do not understand why. Here is my code:

ode45 second order differential equation

function yp = dae_normale(t,y)
yp = zeros(2,1);
yp(1) = y(2);
yp(2) = 4*y(2) + 1/(5*y(2) - 2*y(1) ) - 7*y(1);


ode45 second order differential equation run

[t,y] = ode45('dae_normale',[1,5],[1,1]);
[t,y(:,1)]
plot(t,y(:,1))


DAE ode15s

function out = dae(t,y)
out = [y(2)
4*y(2) + 1/y(3) - 7*y(1)
y(3) - 5*y(2) + 2*y(1) ];


DAE ode15s run

y0 = [1; 1; 3];
M = [1 0 0; 0 1 0; 0 0 0];
options = odeset('Mass',M);
[t,y] = ode15s(@dae,[1 5],y0,options);
[t,y(:,1)]
plot(t,y(:,1))


opts=odeset('RelTol', 1e-4, 'AbsTol', 1e-7);