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Congratulations Andrei for another very good solution.
The 4 functions are: A(x) = \sin x,\quad B(x) = 1 - \cos x,\quad C(x) = \log (1+x), \quad D(x) = 1- {1\over (1-x)}.
Build series for the sine and cosine functions by adding one term at a time, alternately making the approximation too big then too small but getting ever closer.
Look at the advanced way of viewing sin and cos through their power series.