Question

Se considera progresia aritmetica in care a1=3 si a3=7. Sa se calculeze suma primilor 10 termeni ai progresiei.

Answer 1

a3=a1+2r
3+2r=7
2r=4 ⇒r=2

S₁₀=$\frac{(a1+a10) *10}{2}$=(a1+a1+9r)*5=(2a1+9r)*5=(2*3+9*2)*5=(6+18)*5=24*5=120