a1=3a2=8 a3=15 a4=24a2-a1=8-3=5=2*2+1a3-a2=15-8=7=2*3+1a4-a3=24-15=9=2*4+1.an-a(n-1)=2*n+1相加得an-a(n-1)+.+a4-a3+a3-a2+a2-a1=2*(n+.4+3+2)+n-1an-a1=2(n+2)(n-1)/2+n-1=n^2+n-2+n-1=n^2+2n-3an=n^2...