Their number is the angle in radians for which you want the cosine. If the angle is in degrees, then multiply the angle by PI()/180 or use the RADIANS function to convert the angles to radians
How the COS function Works
In Excel, if you want to measure cosine of a given angle then you can use COS function but the condition is that angel must be in radian not in degree. When we are solving trigonometric expressions like sine, cosine, and tangent, it’s very important to realize that Excel uses radians, not a degree to perform these calculations! If the angle is in the degree you must first convert it to radian.
Figure1: Example of returns the cosine of the angle given in radians
To find the result of the cosine of the angle given in radians, you can use the COS function. In the example selected cell shown, the formula in C5 is:
After, solve COS function returns a numeric value.
Converting angle from Degrees to Radians:
In Excel, if your given angle is in degrees, you want to convert the angle into radians before supplying it to the COS function.
We can use Excel radians function:
As we used in the above example