1)а13=7+4*12=55
S13=7+55/2*13=403
2)а40=2+2*39=80
S40=2+80/2*40=1640
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1)а13=7+4*12=55
S13=7+55/2*13=403
2)а40=2+2*39=80
S40=2+80/2*40=1640