ecuatii.cpp: In function 'void calcul(char*, char*)': ecuatii.cpp:42: warning: NULL used in arithmetic ecuatii.cpp:49: warning: NULL used in arithmetic ecuatii.cpp:68: warning: NULL used in arithmetic ecuatii.cpp:97: warning: NULL used in arithmetic ecuatii.cpp:125: warning: NULL used in arithmetic ecuatii.cpp:132: warning: NULL used in arithmetic ecuatii.cpp:151: warning: NULL used in arithmetic ecuatii.cpp:180: warning: NULL used in arithmetic