Question

Aflați suma primilor 10 termeni ai progresiei aritmetice:

Answer 1

[tex]\it \ \div\ \ a_1=2, \ \ a_2=5 \\ \\ r= a_2-a_1=5-2=3 \\ \\ a_{10} = a_1 + 9r = 2 + 9\cdot3 = 2+27 = 29 \\ \\ \\ S_{10}=\dfrac{(a_1+a_{10})\cdot10}{2} = (2+29)\cdot5 =31\cdot5=155[/tex]