Casio fx-100AU COS calculation error

Question

I am calculating the following on my Casio fx-100AU:

7.5cos((2pi/365)*24)+21.5 -> 28.99980502, which is incorrect it should be 28.368982162

I narrowed down the error to the cos calculation...

2pi/365 * 24 = 0.4131409517, the cos result for 0.4131409517 on my calculator is 0.9999740032, where as an online calculator is 0.91586428826.

My question is this normal for a calculation to be so far out for this type of calculation?

Answer 1

Problem: Your calculator uses incorrect angle unit, see this part of user's guide: users guide

Solution: Press the [MODE] key and then select [1] for degrees or [2] for radians.

This can be confirmed with below calculation with Pari/GP:

Reading GPRC: /etc/gprc
GPRC Done.

                  GP/PARI CALCULATOR Version 2.13.1 (released)
          amd64 running linux (x86-64/GMP-6.2.1 kernel) 64-bit version
      compiled: Jan 25 2021, gcc version 10.2.1 20210110 (Debian 10.2.1-6)
                            threading engine: pthread
                 (readline v8.1 enabled, extended help enabled)

                     Copyright (C) 2000-2020 The PARI Group

PARI/GP is free software, covered by the GNU General Public License, and comes
WITHOUT ANY WARRANTY WHATSOEVER.

Type ? for help, \q to quit.
Type ?17 for how to get moral (and possibly technical) support.

parisize = 8000000, primelimit = 500000
? cos(0.4131409517)
%1 = 0.91586428826927817341306651006376638197

? cos(0.4131409517*Pi/180)
%2 = 0.99997400323256593026613148347067028673