acos vs atan

{"ScriptPreparationCode":"var N = 1000000;\r\nvar x = new Float32Array(N);\r\nvar y = new Float32Array(N);\r\n\r\nfor (var i = 0; i \u003C N; \u002B\u002Bi ) {\r\n\tx[i] = 100* Math.random();\r\n\ty[i] = 100* Math.random();\r\n}\r\n","TestCases":[{"Name":"acos","Code":"for (var i = 0; i \u003C N; \u002B\u002Bi) { Math.acos(x[i] / Math.sqrt(x[i]*x[i] \u002B y[i]*y[i])); }","IsDeferred":false},{"Name":"atan","Code":"for (var i = 0; i \u003C N; \u002B\u002Bi) { Math.atan(x[i], y[i]); }","IsDeferred":false}]}