$(y-5)^5$
$=C_5^0.y^5-C_5^1.y^4.5+C_5^2.y^3.5^2-C_5^3.y^2.5^3+C_5^4.y.5^4-C_5^5.5^5$
$=y^5-25y^4+250y^3-1250y^2+3125y-3125$
$\Big(x-\dfrac{2}{x}\Big)^{16}$
$=\sum\limits_{k=0}^{16}C_{16}^k.x^{16-2k}.(-2)^k$
$\Rightarrow 16-2k=8\Leftrightarrow k=4$
Vậy số hạng là $C_{16}^4.(-2)^4=29120$