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a₃=a₁+2r
mamy a₃=21 i r=8
21=a₁+2*8
21=a₁+16
a₁=21-16
a₁=5
liczymy a₁₃ ze wzoru a₁₃=a₁+12r
a₁₃=5+12*8
a₁₃=5+96
a₁₃=101
liczymy S₁₃ ze wzoru Sn=((a₁+an):2)*n gdzie an=a₁₃ a n=13
S₁₃=((5+101):2)*13
S₁₃=(106:2)*13
S₁₃=53*13
S₁₃=689