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3,6,10,15,21. a2-a1=3,a3-a2=4 a4-a3=5 a5-a4=6 . an-a(n-1)=n+1各式相加a2-a1+a3-a2+a4-a3+…+an-a(n-1)=3+4+5+6+..+(n+1)an-a1=(3+n+1)(n-1)/2=(n+4)(n-1)/2 an=3+(n+4)(n-1)/2
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