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1/2*(a₁+17)*8=52 /:4
a₁+17=13
a₁=13-17
a₁=-4
an=a₁+r(n-1)
a₈=a₁+7r
7r=17-(-4)
7r=17+4=21 /:7
r=3
a₃=a₁+2r
a₃=-4+2*3=-4+6
a₁=2
Odp. C