а9=а1+8d
a9=13+8*(-2)
a9=-3
S18=(a1+a18)*n/2
a18=a1+17d=13+17*(-2)=-21
S18=-8*18/2=-72
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а9=а1+8d
a9=13+8*(-2)
a9=-3
S18=(a1+a18)*n/2
a18=a1+17d=13+17*(-2)=-21
S18=-8*18/2=-72