Specification for this function does not specify wheter angle arguments are expressed as radians or degrees.
To specify if argument should be expressed as radians or degrees..
Atan() function Part 4, section 3.17.7.14
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Proposed Disposition of DIS 29500 Comment CL-0022 (Modified: 2007-10-17) The unit of arguments and return values that represent angles is the radian. The formulas having such arguments or return values are ACOS, ASIN, ATAN, ATAN2, COS, SIN, and TAN. This will be made explicit in the descriptions of each of these formulas by the addition of the text ", in radians," or ", in radians", as appropriate, as highlighted in the samples below: 3.17.7.50 COS Syntax: COS ( x ) Arguments: Name Type Description x number The value, in radians, whose cosine is to be determined. 3.17.7.4 ACOS Syntax: ACOS ( x ) Return Type and Value: number The arc cosine of x , in radians . Similar Comments: AU-0025 , CA-0048 , CL-0017 , CL-0018 , CL-0019 , CL-0020 , CL-0021 , CL-0023 , CL-0159 , CL-0166 , CL-0178 , CL-0182 , CO-0159 , CO-0162 , CO-0163 , CO-0164 , CO-0174 , CO-0201 , CO-0202 , DK-0161 , ECMA-0024 , FR-0440 , FR-0444 , FR- 0445 , FR-0446 , FR-0482 , FR-0490 , FR-0505 , GB-0371 , GB-0374 , GB-0375 , GB-0376 , GB-0386 , GB-0424 , GB-0431 , GR-0103 , IE- 0007 , IN-0073 , PH-0002 , US-0173 , US-0176 , US-0177 , US-0178 , US-0185 , US-0220 , US-0227 , UY-0009
Dupe of Ecma 24