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#### 96.2.7 Contract atan Functions

Function: atan_contract (r)

  The function atan_contract(r) contracts atan functions. We assume: \(|r| < {\pi\over 2}.\)

  Examples:

      (%i1) load(trigtools)$

  1.     (%i2) atan_contract(atan(x)+atan(y));
         (%o2)                          atan(y) + atan(x)
         (%i3) assume(abs(atan(x)+atan(y))<%pi/2)$
         (%i4) atan(x)+atan(y)=atan_contract(atan(x)+atan(y));
                                                          y + x
         (%o4)                  atan(y) + atan(x) = atan(-------)
                                                         1 - x y

  2.     (%i5) atan(1/3)+atan(1/5)+atan(1/7)+atan(1/8)$ %=atan_contract(%);
                                1         1         1         1    %pi
         (%o6)             atan(-) + atan(-) + atan(-) + atan(-) = ---
                                3         5         7         8     4

  3. Machin’s formulae

         (%i7) 4*atan(1/5)-atan(1/239)=atan_contract(4*atan(1/5)-atan(1/239));
                                          1          1     %pi
         (%o7)                     4 atan(-) - atan(---) = ---
                                          5         239     4

  4. see http://en.wikipedia.org/wiki/Machin-like_formula

         (%i8) 12*atan(1/49)+32*atan(1/57)-5*atan(1/239)+12*atan(1/110443)$
         %=atan_contract(%);
                         1             1             1               1       %pi
         (%o9)   12 atan(--) + 32 atan(--) - 5 atan(---) + 12 atan(------) = ---
                         49            57           239            110443     4

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