n(n+1)(n+2)(n+3)+1=(n^2+3n+1)^2
n(n+1)(n+2)(n+3)+1
=[n(n+3)][(n+1)(n+2)]+1
=(n^2+3n)(n^2+3n+2)+1
=(n^2+3n)^2+2(n^2+3n)+1
=(n^2+3n+1)^2
a²+5a+5
(a+1)(a+2)(a+3)(a+4)+1
=(a+1)(a+4)*(a+2)(a+3)+1
=(a²+5a+4)(a²+5a+6)+1
设a²+5a为X
(X+4)(X+6)+1
=X²+10X+25
(X+5)²
即(a+1)(a+2)(a+3)(a+4)+1=(a²+5a+5)
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