### 4.4.6 `fsolve` or `nSolve` with the option `steffenson_solver`

The Steffenson method is generically the fastest method.
It combines the Newton method with a "delta-two" Aitken acceleration : with the Newton method, we obtain the sequence xi and the convergence acceleration gives the Steffenson sequence

Ri =xi −

(xi+1 − xi)2
(xi+2 − 2 xi+1 + xi)

Input :

`fsolve(cos(x)=x,x,0,steffenson_solver)`

Output :

`0.739085133215`