44. Khai triển các biểu thức sau:
a) $(x – 2y)^{4}$;
b) $(–3x – y)^{5}$.
Bài Làm:
a) $(x – 2y)^{4} = x^{4} + 4\times x^{3}\times (–2y) + 6\times x^{2}\times (–2y)^{2}+ 4\times x\times (–2y)^{3} + (–2y)^{4}$
$= x^{4} – 8x^{3}y + 24x^{2}y^{2} – 32xy^{3} + 16y^{4}$.
b) $(–3x – y)^{5}$
= $(–3x)^{5} + 5\times (–3x)^{4}\times (–y) + 10\times (–3x)^{3}\times (–y)^{2} + 10\times (–3x)^{2}\times (–y)^{3} + 5\times (–3x)\times (–y)^{4} + (–y)^{5}$.
= $–243x^{5} – 405x^{4}y – 270x^{3}y^{2} – 90x^{2}y^{3} – 15xy^{4} – y^{5}$.