Trả lời :

B = x⁶ - 20x⁵ - 20x⁴ - 20x³ - 20x² - 20x - 3    

Thay 20 = x - 1 

=> B = x⁶ - (x - 1) . x⁵ - (x - 1) . x⁴ - (x - 1) . x³ - (x - 1) . x² - (x - 1) . x - 3    

=> B = x⁶ - x⁶ + x⁵ - x⁵ + x⁴ - x⁴ + x³ - x³ + x² - x² + x - 3

=> B = x - 3

Thay x = 21 => B = 21 - 3 = 19

Vậy B = 19.

giúp mình nha người đầu tiên

\(\frac{5}{2y^2+6y}-\frac{4y-3y^3}{y^3-9y}-3\)

ĐKXĐ : \(\hept{\begin{cases}y\ne0\\y\ne\pm3\end{cases}}\)

\(=\frac{5}{2y\left(y+3\right)}-\frac{y\left(4-3y^2\right)}{y\left(y^2-9\right)}-3\)

\(=\frac{5}{2y\left(y+3\right)}-\frac{4-3y^2}{y^2-9}-3\)

\(=\frac{5}{2y\left(y+3\right)}-\frac{4-3y^2}{\left(y-3\right)\left(y+3\right)}-3\)

\(=\frac{5\left(y-3\right)}{2y\left(y-3\right)\left(y+3\right)}-\frac{2y\left(4-3y^2\right)}{2y\left(y-3\right)\left(y+3\right)}-\frac{3\cdot2y\left(y-3\right)\left(y+3\right)}{2y\left(y-3\right)\left(y+3\right)}\)

\(=\frac{5y-15}{2y\left(y-3\right)\left(y+3\right)}-\frac{8y-6y^3}{2y\left(y-3\right)\left(y+3\right)}-\frac{6y\left(y^2-9\right)}{2y\left(y-3\right)\left(y+3\right)}\)

\(=\frac{5y-15-8y+6y^3-6y^3+54y}{2y\left(y-3\right)\left(y+3\right)}\)

\(=\frac{51y-15}{2y\left(y-3\right)\left(y+3\right)}=\frac{3\left(17y-5\right)}{2y\left(y-3\right)\left(y+3\right)}\)