Hãy khai triển: a) (x – y)^6; b) (1 + x)^7.
Giải thích
Hướng dẫn giải
a) (x – y)6
=C60x6+C61x5(−y)+C62x4(−y)2+C63x3(−y)3
+C64x2(−y)4+C65x(−y)5+C66(−y)6
=x6−C61x5y+C62x4y2−C63x3y3+C64x2y4−C65xy5+y6
=x6−6x5y+15x4y2−20x3y3+15x2y4−6xy5+y6.
b)(1 + x)7
=C7017+C7116x+C7215x2+C7314x3
+C7413x4+C7512x5+C761x6+C77x7
= 1 + 7x + 21x2 + 35x3 + 35x4 + 21x5 + 7x6 + x7.