Add COS function description

docs/src/functions/math-and-trigonometry.md

@@ -29,7 +29,7 @@ You can track the progress in this [GitHub issue](https://github.com/ironcalc/Ir
 | CEILING.PRECISE | <Badge type="info" text="Not implemented yet" /> | –             |
 | COMBIN          | <Badge type="info" text="Not implemented yet" /> | –             |
 | COMBINA         | <Badge type="info" text="Not implemented yet" /> | –             |
-| COS             | <Badge type="tip" text="Available" />          | –             |
+| COS             | <Badge type="tip" text="Available" />          | [COS](math_and_trigonometry/cos) |
 | COSH            | <Badge type="tip" text="Available" />          | –             |
 | COT             | <Badge type="info" text="Not implemented yet" /> | –             |
 | COTH            | <Badge type="info" text="Not implemented yet" /> | –             |

docs/src/functions/math_and_trigonometry/cos.md

@@ -3,9 +3,41 @@ layout: doc
 outline: deep
 lang: en-US
 ---
-
+# COS function
-# COS
-
 ::: warning
-🚧 This function is implemented but currently lacks detailed documentation. For guidance, you may refer to the equivalent functionality in [Microsoft Excel documentation](https://support.microsoft.com/en-us/office/excel-functions-by-category-5f91f4e9-7b42-46d2-9bd1-63f26a86c0eb).
+**Note:** This draft page is under construction 🚧
-:::
+:::
+## Overview
+COS is a function of the Math and Trigonometry category that calculates the trigonometric cosine of an angle, returning a value in the range [-1, +1].
+## Usage
+### Syntax
+**COS(<span title="Number" style="color:#1E88E5">angle</span>) => <span title="Number" style="color:#1E88E5">cos</span>**
+### Argument descriptions
+* *angle* ([number](/features/value-types#numbers), required). The angle whose cosine is to be calculated, expressed in radians. To convert between degrees and radians, use the relation below. Alternatively, use the [DEGREES](/functions/math_and_trigonometry/degrees) or [RADIANS](/functions/math_and_trigonometry/radians) functions.
+$$
+1~\:~\text{degree} = \dfrac{\pi}{180} = 0.01745329252~\text{radians}
+$$
+
+### Additional guidance
+None.
+### Returned value
+COS returns a unitless [number](/features/value-types/#numbers) that is the trigonometric cosine of the specified angle.
+### Error conditions
+* In common with many other IronCalc functions, COS propagates errors that are found in its argument.
+* If no argument, or more than one argument, is supplied, then COS returns the [`#ERROR!`](/features/error-types.md#error) error.
+* If the value of the *angle* argument is not (or cannot be converted to) a [number](/features/value-types#numbers), then COS returns the [`#VALUE!`](/features/error-types.md#value) error.
+* For some argument values, COS may return a [`#DIV/0!`](/features/error-types.md#div-0) error.
+<!--@include: ../markdown-snippets/error-type-details.txt-->
+## Details
+* The COS function utilizes the *cos()* method provided by the [Rust Standard Library](https://doc.rust-lang.org/std/).
+* The figure below illustrates the output of the COS function for angles $x$ in the range -2$\pi$ to +2$\pi$ radians.
+<center><img src="/functions/images/cosine-curve.png" width="350" alt="Graph showing cos(x) for x between -2π and +2π radians."></center>
+
+## Examples
+[See some examples in IronCalc](https://app.ironcalc.com/?example=cos).
+
+## Links
+* For more information about trigonometric cosine, visit Wikipedia's [Sine and cosine](https://en.wikipedia.org/wiki/Sine_and_cosine) page.
+* See also IronCalc's [ACOS](/functions/math_and_trigonometry/acos), [SIN](/functions/math_and_trigonometry/sin) and [TAN](/functions/math_and_trigonometry/tan) functions.
+* Visit Microsoft Excel's [COS function](https://support.microsoft.com/en-gb/office/cos-function-0fb808a5-95d6-4553-8148-22aebdce5f05) page.
+* Both [Google Sheets](https://support.google.com/docs/answer/3093476) and [LibreOffice Calc](https://wiki.documentfoundation.org/Documentation/Calc_Functions/COS) provide versions of the COS function.