What Does This Error Mean?

HP Prime CAS errors usually mean the expression was entered with incorrect CAS syntax, or CAS mode settings conflict with the operation. Confirm you are in the CAS app (not the Home screen), use exact fractions instead of decimals, and ensure the variable has not been previously assigned a value in CAS memory.

Affected Models

HP Prime G1
HP Prime G2

Common Causes

Expression contains a previously assigned CAS variable (e.g. X=3 overrides symbolic X)
Using decimal numbers in CAS — CAS prefers exact integers and fractions for symbolic computation
Attempting a CAS function on the Home screen instead of the CAS screen
CAS settings (complex mode, angle unit) incompatible with the problem
CAS memory corrupted — requires a CAS memory reset

How to Fix It

Open the CAS screen, not the Home screen.

On the HP Prime, press the CAS key to enter the CAS (Computer Algebra System) screen. CAS functions like factor(), solve(), and integral() only work correctly in CAS mode. Entering them on the Home screen either gives a numeric approximation or an error.
Clear assigned variables.

In CAS, if you previously stored a value in X (e.g. 5 → X), the variable X is no longer treated symbolically — it holds the number 5. To restore symbolic computation, type purge(X) in CAS to remove the assignment. Alternatively, go to CAS > Settings and select Reset CAS Variables.
Use exact values, not decimals.

CAS performs symbolic algebra and works best with exact values. Instead of entering 0.5, enter 1/2. Instead of 3.14159, use pi. Decimal approximations cause CAS to switch to numeric mode, which can return unexpected results or errors for symbolic operations.
Check the CAS settings.

Press Shift + Home > CAS Settings. Confirm the angle unit matches your problem (Radians or Degrees). For problems involving real numbers only, ensure Complex Mode is OFF — with it ON, the CAS may return complex number results for operations like sqrt(-1) that appear incorrect if you expected real results only.
Reset the CAS memory.

Press Shift + Home > Reset > Reset CAS. This clears all CAS variable assignments, custom functions, and stored CAS results. After a CAS reset, all symbolic variables are clean and unassigned. This resolves errors caused by accumulated conflicting variable assignments.

When to Call a Professional

CAS errors are software and input issues — no hardware service is needed on the HP Prime.