@tomleslie Yes,Thanks!

@Kitonum Thanks!

 big thx guys!

@Preben Alsholm 

Could you explain to me why we define Fdiff2, F1diff2 since the original system is changed ?



Just this:

is not:

Maybe I'm wrong course

if u can.
thx.
doc.mw

@Preben Alsholm BIG thx man:)))

@Preben Alsholm 

thx , but i have a new problem:)))

i tried to do so 

Fepta := F(.234);

F1epta := F1(.234);

Fdiffepta := Fdiff(.234);

F1diffepta := F1diff(.234)

Sysq := ((9/10)*F1epta*Fepta*F1diffepta-(3/10)*Fdiffepta*F1epta*F1epta+(3/2)*F1epta)*Fepta = (1/5)*Fepta*delta1(x)-(3/10)*u123(x);

Sysqq := u123(x)*(eval(Fdiffepta))+(eval(F1epta))*(diff(delta1(x), x))+delta1(x)*F1diffepta+Fepta*(diff(u123(x), x)) = 0;

SystemSec := Sysq, Sysqq;

SYS22 := dsolve({SystemSec, delta1(0) = 10^(-9), u123(0) = 10^(-9)}, {delta1(x), u123(x)}, type = numeric);

and i have a error :

Error, (in dsolve/numeric/DAE/checkconstraints) the initial conditions do not satisfy the algebraic constraints
error = .211e17, tolerance = .150e11, constraint = -21105640878048057+2150000000000000*delta1(x)-15000000000000000*u123(x)

 doc.mw