a)
3,8,13,18...... n₁₀₀= 498
an= 5*n -2
a₁₀₀= 5*100-2=500-2=498
b) 3+8+13+18+....n₅₀=3+8+13+18+...+248=(248+3)*50/2=251*25=6275
n₅₀=5*50-2=250-2=248
alta modalitate:
b) 3+8+13+18+....n₅₀=3+8+13+18+...+248= 5*1-2+5*2-2+5*3-2+.. ...+5*50-2=5*(1+2+3+...50)+ (-2)*50= 5*50*51/2 - 100=6375-100= 6275
