Răspuns:
c) 16+17+18...+95
= ( 1+2+3+4+....+16+17+18+....+95) - (1+2+3+4+....+15)=
= (95·96)/2 - (15·16)/2=95·48-15·8=4560-120=4440
d) 15+16+..+74 = (1+2+3+...+14+15+16+....+74)-(1+2+3+...+14)=
=(74·75)/2 - (14·15)/2=37·75-7·15=2775-105=2670
e) 25+30+35+...+220= 5( 5+6+7+...+44) =5[( 1+2+3+4+5+6+7+....+44)-(1+2+3+4)]=5[(44·45)/2-10]=5·(22·45-10)=5(990-10)=5·980=4900
Explicație pas cu pas:
