System of non-linear equations

[parisse]

Didi you try on emulator or real calc? With inversion of jacobian after substitution (new code) it takes about 1/100s on the emulator.
The starting point is important for iterative methods: sometimes it converges, sometimes not...
For fsolve([cos(x)-x,x-y],[x=0..1,y=0..1]), the solver probably restarted from a random point, after an initial try with an invalid guess.