(x-1）（x+1)(x+3)(x+5)+1
=(x^2+4x+5)(x^2+4x+3)+1
=(x^2+4x)^2+8(x^2+4x)+15+1
=(x^2+4x)^2+8(x^2+4x)+16
=(x^2+4x+4)^2
=(x+2)^4