Question

Sa se calculeze suma primelor zece termeni ai unei progresii aritmetice daca a2=3 si a5=15

Answer 1

a2=3; a5=15

a2+3r=a5

3+3r=15 =>r=12/3

r=4

a1=a2-r

a1=-1

a10=-1+9r=35

S10=10(-1+35)/2

S10=170

Answer 2

a5=a2+3r <=> 12=3r <=> r=4
S10=(a1+a10)*10/2=5(a1+a10)=5()a1=a2-r=3-4=-1a10=a1+9r=-1+36=35S10=5(-1+35)=5*34=170