The + operator adds two integers in ℤ/pℤ or two polynomials in ℤ/pℤ[x]. (See also Section 7.3.2.) For polynomial expressions, use the normal command to simplify.
For integers in ℤ/pℤ:
| 3%13+10%13 | 
| 
 | 
For polynomials with coefficients in ℤ/pℤ:
| normal((11*x+5)%13+(8*x+6)%13) | 
or:
| normal((11% 13*x+5%13)+(8% 13*x+6%13)) | 
| 
 |