1) an = a1 + d(n-1)
a80 = 74 -6(79) = -400
2)Sn = (a1+an)*n/2
a18 = 74 -6*17 = -28
S18 = (74-28)*18/2 = 414
1)a80=a1+d879=74-6*79=74-474=-400
2)s18=(a1+a18)/2*8n=(74-28)/2*18=23*18=414
1) an = a1 + d(n-1)
a80 = 74 -6(79) = -400
2)Sn = (a1+an)*n/2
a18 = 74 -6*17 = -28
S18 = (74-28)*18/2 = 414
1)a80=a1+d879=74-6*79=74-474=-400
2)s18=(a1+a18)/2*8n=(74-28)/2*18=23*18=414