What Does This Error Mean?

If a Casio calculator gives wrong answers for trigonometry, the angle unit is almost certainly set to Radians instead of Degrees (or vice versa). Check the top of the screen for a small R or D indicator. Press SHIFT + MENU (Setup) and change the angle unit to match your problem.

Affected Models

Casio fx-991EX
Casio fx-82ES Plus
Casio fx-CG50
Casio fx-9750GIII
Casio ClassWiz fx-991CW
All Casio scientific calculators

Common Causes

Angle unit set to Radians when problem requires Degrees (or vice versa)
Calculator in STAT or TABLE mode — standard arithmetic inputs behave differently
Decimal notation set to comma instead of period (or vice versa) causing input misreads
Number format set to Fix or Sci mode showing a fixed number of decimal places
Complex number mode active — answers include imaginary components

How to Fix It

Check and change the angle unit.

Look at the top of the Casio screen for a small indicator: D = Degrees, R = Radians, G = Gradians. For most school problems, you need Degrees. To change: press SHIFT + MENU (Setup), then select Angle Unit, and press 1 for Degrees. This fixes incorrect sin, cos, and tan answers instantly.
Return to standard COMP mode.

Press MENU and select 1 for COMP (Computation) mode. If the calculator is in STAT (statistics), TABLE, or other modes, standard arithmetic entries will give unexpected results. COMP mode is the default for everyday calculations.
Check the number display format.

If answers show more or fewer decimal places than expected, the display format may be set to Fix, Sci, or Eng. Press SHIFT + MENU (Setup) > Display and select Norm 1 or Norm 2 for standard notation. Fix 2, for example, always shows exactly 2 decimal places.
Check the decimal separator setting.

Some Casio models allow the decimal point to be set as a comma (European format). If your calculator shows 3,14 instead of 3.14, the separator setting has changed. Press SHIFT + MENU (Setup) > Decimal Point and select Dot (.) for standard notation.
Perform a full reset.

Press SHIFT + 9 (RESET) > 3 (All) > = (YES) to restore all settings to factory defaults. This clears all mode changes including angle unit, display format, and complex number settings. Note: this also erases stored programs, custom functions, and STAT data.

When to Call a Professional

Mode and settings issues require no hardware repair — they are always correctable through the Casio settings menu.