Question

fie sirul de numere 3,8,13,18... Determina numarul de pe pozitia 100.Afla suma primilor 50 de termeni din sir.

Answer 1

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