25. Calculează: a) 20-19-18-17+16-15+...+2-1= b) 111-110-109-108 + ... +3-2-1= c) 300 +299 - 298–297 +290 +295-294-293+ ... +4+3-2-1 = d) 100+99-98-97 +99+98-97-96+ ... +5+4-3-2 +4+3-2-1=

Ajutor urgent​