a1=2,a2=4,a3=7,a4=11,.发现a2-a1=2a3-a2=3a4-a3=4.所以递推an-a(n-1)=n所以an=(an- an-1)+(an-1-an-2)+(an-2 - an-3)+.+(a2-a1)+a1=n+(n-1)+(n-2)+.+2+2 =(n+2)(n-1)/2+...