а3=5, а11=17. Найдите а7
a_(3)+a_(11)=a_(1)+2*d+a_(1)+10*d=2a_(1)+12d=2([b]a_(1)+6d[/b])=2[b]a_(7)[/b] a_(3)+a_(11)=5+17=22 2a_(7)=22 a_(7)=11