
Numerical Solution of Differential-Algebraic Equations - wfunction
http://reference.wolfram.com/language/tutorial/NDSolveDAE.html
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japaget
The example:

    
    
        (1) x''(t) + y(t) = cos(t)
        (2) x(t) = cos(t)
    

is trivially solvable by differentiating (2) twice and substituting into (1)
to give:

    
    
        -cos(t) + y(t) = cos(t)
    

or

    
    
        y(t) = 2 cos(t)
    

Together with (2) that gives the solution; there is no need to use advanced
techniques to solve this particular example.

