an-a(n-1)=3n-1
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a2-a1=3*2-1=5
a3-a2=3*3-1=8
a4-a3=3*4-1=11
...
an-a(n-1)=3n-1
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an-a1=5+8+11+...+3n-1=(n-1)[5+(3n-1)]/2=(3n^2+n-4)/2
an-a1=(3n^2+n-4)/2
an=a1+(3n^2+n-4)/2=2+(3n^2+n-4)/2=(3n^2+n)/2
an=(3n^2+n)/2
an=n(3n+1)/2
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an-a1=(a2-a1)+(a3-a2)+...+[an-a(n-1)]=5+8+11+...+(3n-1)
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an-a1=5+8+11+...+(3n-1)
an-a1==(3n^2+n-4)/2
an=a1+(3n^2+n-4)/2
an=n(3n+1)/2
2)an/a(n-1)=(n-1)/n
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an/a1=(a2/a1)*(a3/a2)*(a4/a3)*...*[an/a(n-1)]=(1/2)*(2/3)*(3/4)*...*[(n-1)/n]
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an/a1=1/n
an=a1/n
an=1/n
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