问题
解答题
观察下面各式:
12+(1×2)2+22=(1×2+1)2;
22+(2×3)2+32=(2×3+1)2;
32+(3×4)2+42=(3×4+1)2;…
(1)写出第2006个式子;
(2)写出第n个式子,并证明你的结论。
答案
解:(1)第2006个式子即当n=2006时,有20062+(2006×2007)2+20072=(2006×2007+1)2;
(2)第n个式子为n2+[n(n+1)]2+(n+1)2=[n(n+1)+1]2,
证明如下:
因为n2+[n(n+1)]2+(n+1)2
=n2+n2(n+1)2+(n2+2n+1)
=n2+n4+2n3+n2+n2+2n+1
=n4+2n3+3n2+2n+1,
且[n(n+1)+1]2=[n(n+1)]2+2[n(n+1)]·1+12
=n2(n+1)2+2n(n+1)+1
=n2(n2+2n+1)+2n2+2n+1
=n4+2n3+n2+2n2+2n+1
=n4+2n3+3n2+2n+1,
所以n2+[n(n+1)]2+(n+1)2
=[n(n+1)+1]2。