Note: Since is not known, the RHS of (3.9) cannot be evaluated. Two approaches are possible. One is to attempt to solve the nonlinear equation (3.9) for . This is called an implicit method. Alternatively, one can estimate by another method, such as the Euler method, and get an explicit method. Then we get the formula
is not known, the RHS of (3.9) cannot be evaluated. Two approaches are possible. One is to attempt to solve the nonlinear equation (3.9) for 
