an=an-1+n(n+1)/2 a1=1 an-1=an-2+(n-1)(n-1+1)/2 an-2=an-3+(n-2)(n-2+1)/2 . . . . a4=a3+4*(4+1)/2 a3=a2+3(3+1)/2 a2=a1+2*(2+1)/2 a1=1*(1+1)/2 代入得，an=1*(1+1)/2+2*(2+1)/2+3*(3+1)/2+4*(4+1)/2+.....+n(n+1)/2 =[(1^2+2^2+3^2+4^2+....+n^2)+(1+2+3+4+....+n)]/2 =[(1^2+2^2+3^2+4^2+....+n^2)+n(n+1)/2]/2
答：解：（n+1)/n×（n+1)=(n+1)²/n=(n²+2n+1)/n=(n+1)/n+(n²+n)/n =(n+1)/n+(n...详情>>
答：One Sunday, Mark decided to go sailing in his boat with his friend Dan, but Dan ...详情>>
