a1=6a2=10a3=16a4=24a2-a1=10-6=4=2*2a3-a2=16-10=6=2*3a4-a3=24-16=8=2*4*******an-a(n-1)=2*n相加得an-a1=2*(2+3+4+.n)=2*(n+2)*(n-1)/2=(n+2)*(n-1)an=(n+2)(n-1)+a1=n^2+n-2+6=n^2+n+4