How about showing us what your calc syntax looks like, and providing some example data with correct and incorrect answers? I would have thought:

If T value is in cell A1, and the angle A is in cell B1, then the formula for R for example in cell C1 = 2*A1*COS(B1/2) would be what you want.

Are you sure the calculator answer is correct? Angle A measured in degrees or radians? Calculator correctly handles the sequence of operations?

You could try breaking down the formula and calculating each part separately, e.g. do you get the same answer from calculator and calc for just COS(A/2) aka =COS(B1/2)?

Edit:

I just found this (https://help.libreoffice.org/Calc/Mat...):

COS
Returns the cosine of the given angle (in radians).

Syntax
COS(Number)

Returns the (trigonometric) cosine of Number, the angle in radians.

To return the cosine of an angle in degrees, use the RADIANS function.

Examples

=COS(PI()*2) returns 1, the cosine of 2*PI radians.

=COS(RADIANS(60)) returns 0.5, the cosine of 60 degrees.

So maybe what you need is:

=2*A1*COS(RADIANS(B1/2))