(x-4)2 - 2(x-4)(x+5) + (x+5)2
=(x2 - 2.x.4 + 42) - 2(x2+5x-4x-20)+(x2+2.x.5+52)
=x2 - 8x + 16 - 2x2-10x+8x+40+x2+10x+25
=(x2 - 2x2 +x2)+(- 8x+8x+10x)+(16+40+25)
=0+0+81=81
Chúc bn học tốt
\(\left(x-4\right)^2-2\left(x-4\right)\left(x+5\right)+\left(x+5\right)^2\)
\(=\left[\left(x-4\right)-\left(x+5\right)\right]=\left(x-4-x-5\right)^2=\left(-1\right)^2=1\)