6, 9, 13, 18. The general formula of this sequence

a2-a1=3
a3-a2=4
a4-a3=5
……
an-a(n-1)=n+1
Add left and right separately
an-a1=3+4+5++n+(n+1)=(n-1)(n+4)/2
an=(n-1)(n+4)/2+6

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