Ta có : x - y = 9
⇒ x = 9 + y
B = $\frac{4x-9}{3x+y}$ - $\frac{4y+9}{3y+x}$
= $\frac{4.(9+y)-9}{3.(9+y)+y}$ - $\frac{4y+9}{3y + 9 +y}$ (thay x = 9+y vào nhé)
= $\frac{36+4y-9}{27+3y+y}$ - $\frac{4y+9}{4y+9}$
= $\frac{27 + 4y}{27+4y}$ - $\frac{4y+9}{4y+9}$
= 1 - 1
= 0