令t=x-5/2
x-1=t+3/2
x-4=t-3/2
x-2=t+1/2
x-3=t-1/2
(x-1)(x-2)(x-3)(x-4)+k
=(x-1)(x-4)(x-2)(x-3)+k
=(t+3/2)(t-3/2)(t+1/2)(t-1/2)+k
=(t^2-9/4)(t^2-1/4)+k
=t^4-10/4*t^2+9/16+k
显然这个完全平方式是
(t^2-10/8)^2
那么(10/8)^2=9/16+k
k=1
这个平方式是
[(t-5/2)^2-10/8]^2
即[(t-5/2)^2-5/4]^2