10 LET a$="x+x"
20 LET b$="x*x"
30 LET c$="x+x^2"
40 LET f$=c$: REM Assign a$, b$ or c$
150 FOR i=1 TO 5
160 READ x
170 PRINT x;" = ";VAL f$
180 NEXT i
190 STOP
200 DATA 2,5,6,10,100