Now from (6.2) with , we have
, 
Expanding by Taylor series about we get
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(6.5) |
Since these two equations are implicit, we can no longer proceed by successive substitution as done in the case of explicit Runge-Kutta methods earlier. Let us assume instead, that the solutions for and may be expressed in the form
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(6.6) |
Substituting for by (6.6) in (6.5) we get
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