Fixed a bug in Atan. Did not correctly handle when input is a large n…

…egative value. (#135)

* corrected the last digit of a test expected value (sourced by wolframaplpha.com for correctness)

* fix a bug when the values are large negative values and added tests

math/arccosine_test.go

@@ -14,7 +14,7 @@ func TestAcos(t *testing.T) {
 		x, r string
 	}{
 		0: {"-1.00", "3.141592653589793238462643383279502884197169399375105820974944592307816406286208998628034825342117068"},
-		1: {"-.9999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999", "3.141592653589793238462643383279502884197169399375091678839320861357328389398966901647249128623363298"},
+		1: {"-.9999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999", "3.141592653589793238462643383279502884197169399375091678839320861357328389398966901647249128623363299"},
 		2: {"-0.50", "2.094395102393195492308428922186335256131446266250070547316629728205210937524139332418689883561411379"},
 		3: {"0", "1.570796326794896619231321691639751442098584699687552910487472296153908203143104499314017412671058534"},
 		4: {"0.5", "1.047197551196597746154214461093167628065723133125035273658314864102605468762069666209344941780705689"},

math/arctangent.go

@@ -150,6 +150,12 @@ func Atan(z, x *decimal.Big) *decimal.Big {
 		return z
 	}
 
+	//when x <-1 we use -atan(-x) instead
+	if x.Cmp(negone) < 0 {
+		Atan(z, new(decimal.Big).Neg(x))
+		return z.Neg(z)
+	}
+
 	y, ySq, ySqPlus1, segment, halfed := prepAtan(z, x, ctx) // z == y, maybe.
 	result := BinarySplitDynamic(ctx,
 		func(_ uint64) *decimal.Big { return y },