а3=а1+2д
а9=а1+8д
а3+а9=а1+2д+а1+8д=2а1+10д=8
Sn=(2a1+d(n-1))*n/2
S11=(2a1+d(11-1))*11/2=(2a1+10d)11/2=8*11/2=44
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а3=а1+2д
а9=а1+8д
а3+а9=а1+2д+а1+8д=2а1+10д=8
Sn=(2a1+d(n-1))*n/2
S11=(2a1+d(11-1))*11/2=(2a1+10d)11/2=8*11/2=44