Question

a9 = 34
a3+a5 =?
арифметическая последовательность

Answer 1

a9=a3+6d=34
a3+a5=a3+a3+2d=2a3+2d=?

a3=34-6d , 2a3+2d=68-12d+2d=68-10d, d=?
a3+a5=68-10d

Answer 2

$a_{9}= a_{1}+8d\\ a_{1}+8d=34\\\ a_{3}+ a_{15}= a_{1} +2d+ a_{1}+14d=2 a_{1}+16d =2( a_{1} +8d)=2*34=68\\\ a_{3} + a_{15}=68$