SN is the sum of the first n terms of the arithmetic sequence, S4 = 26, sn-4 = 77, Sn = 187, find n

a1+a2+a3+a4=26
an+a(n-1)+a(n-2)+a(n-3)=sn-s(n-4)=110
Two formula addition
（a1+an)+[a2+a(n-1)]+[a3+a(n-2)]+[a4+a(n-3)]=136
Because it's an arithmetic sequence, so
a1+an=a2+a(n-1)=a3+a(n-2)=a4+a(n-3)=136/4=34
And Sn = (a1 + an) n / 2
So 187 = 34n / 2 = 17N
So n = 11

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