(x+2).(x+4).(x+6).(x+8)=\(\left(x+2\right)\left(x+8\right)\left(x+4\right)\left(x+6\right)=\left(x^2+10x+16\right)\left(x^2+10x+24\right)=\left(x^2+10x+21-5\right)\left(x^2+10x+21+4\right)\)
đặt x^2+10x+21=t đi.
\(\left(t-5\right)\left(t+4\right)=t^2-t-20=\left(x^2+10x+21\right)^2-\left(x^2+10x+21\right)-20\)
nhìn là biết dư -20 rồi nha