Question

4. Calculați următoarele sume:
a) 1+2+3+ ... +30;
b) 2+3+4+...+30;
c) 9+10 +11+...+30.​

Answer 1

a) [30(30+1)]:2=(30x31):2=930:2=465

b) 1+2+3+...+30=[30(30+1):2]-1=[(30x31): 2]-1=930:2-1=465-1=464

c) 1+2+3+...+30=[30(30+1)]:2-[8(8+1)]:2=(30x31):2-(8x9):2=465-36=429