$D=x^{10}+20x^9+...+20x^2+20x$
$⇔D=(-24)^{10}+20(-24)^9+...+20(-24)^2+20(-24)$
$⇔D=24^{10}-20.24^9+...+20.24^2-20.24$
$⇒24D=24^{11}-20.24^{10}+...+20.24^3-20.24^2$
$⇒24D+D=24^{11}-20.24^{10}+...+20.24^3-20.24^2+24^10-20.24^9+...+20.24^2-20.24$
$⇒25D=24^{11}-20.24$
$⇒D=\frac{24^{11}-20.24}{25}$
Phần c tương tự hình.