Question

Progresie aritmetica A1=7,a7=37, s10=?

Answer 1

a7=a1+6r
6r=a7-a1
6r=37-7
6r=30
r=5
s10=(a1+a10)×10 /2
apoi aflam a10
a10=a1+9r
a10=7+9×5
a10=7+45
a10=52
apoi inlocuim in formula sumei
s10=(a1+a10)×10 /2
s10=(7+52)×10 /2
s10=59×10 /2
s10=590/2
s10=295

sigur asta este raspunsul