(x+2)(x+4)(x+6)(x+8)+16
=[(x+2)(x+8)[[(x+4)(x+6)]+16
=(x^2+10x+16)(x^2+10x+24)+16
=(x^2+10x)^2+40(x^2+4x)+400
=(x^2+10x+20)^2
(x+2)(x+4)(x+6)(x+8)+16
=[(x+2)(x+8)[[(x+4)(x+6)]+16
=(x^2+10x+16)(x^2+10x+24)+16
=(x^2+10x)^2+40(x^2+4x)+400
=(x^2+10x+20)^2