Initial data commit

Task/Arithmetic-geometric-mean-Calculate-Pi/Sidef/arithmetic-geometric-mean-calculate-pi.sidef

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+func agm_pi(digits) {
+    var acc = (digits + 8);
+
+    local Num!PREC = 4*digits;
+
+    var an = 1;
+    var bn = sqrt(0.5);
+    var tn = 0.5**2;
+    var pn = 1;
+
+    while (pn < acc) {
+        var prev_an = an;
+        an = (bn+an / 2);
+        bn = sqrt(bn * prev_an);
+        prev_an -= an;
+        tn -= (pn * prev_an**2);
+        pn *= 2;
+    }
+
+    ((an+bn)**2 / 4*tn).to_s
+}
+
+say agm_pi(100);