原式=(x+1)(x+4)(x+2)(x+3)+1
=(x²+5x+4)(x²+5x+6)+1
令t=x²+5x
原式=(t+4)(t+6)+1
=t²+10t+24+1
=t²+10t+25
=(t+5)²
t=x²+5x
原式=(x²+5x+5)²
=[(x+1)(x+4)][(x+2)(x+3)]+1
=[(x²+5x)+4][(x²+5x)+6]+1
=(x²+5x)²+10(x²+5x)+24+1
=(x²+5x)²+10(x²+5x)+25
=(x²+5x+5)²
(x+1)(x+2)(x+3)(x+4)+1
=(x^2+5x+4)(x^2+5x+6)+1
=(x^2+5x+4)^2+2(x^2+5x+4)+1
=(x^2+5x+4+1)^2
=(x^2+5x+5)^2