The Secant is the reciprocal of the Cosine.
Therefore, for the rightangled triangle below, the secant of the angle θ is equal to the hypotenuse, h, divided by the adjacent side, a.
I.e. for the triangle above,
The trig. ratios are discussed further on the Wikipedia Trigonometric Ratios PageThe Excel Sec function calculates the secant of a given angle.
Note: the Sec function was only introduced in Excel 2013 and so is not available in earlier versions of Excel.
The syntax of the function is:
Where the number argument is the angle (in radians) that you want to calculate the secant of. This must be between 2^27 and +2^27.
Converting from Degrees to Radians
If your angle is in degrees, you will need to convert it into radians before supplying it to the Sec function. This can be done using the Excel Radians function:
In the following spreadsheet, the Excel Sec function is used to calculate the Secant of four different angles:
Formulas:
 Results:

Note that, in the examples above:
Further details and examples of the Excel Sec function are provided on the Microsoft Office website.
If you get an error from the Excel Sec function, this is likely to be one of the following:
#NUM!    Occurs if the supplied number is less than 2^27 or is greater than 2^27. 
#VALUE!    Occurs if the supplied number is not recognised as a numeric value. 