Sử dụng tam giác Pascal để khai triển: a) (x + y)^7; b) (x – 2)^7.

Vậy:

a) ${\left(x+y\right)}^7=x^7+7x^{6}y+21x^5{y}^2+35x^4{y}^3+35x^3{y}^4+21x^2{y}^5+7x{y}^6+y^7.$

b) ${\left(x-2\right)}^7$

$=x^7+7x^6.\left(-2\right)+21x^5{\left(-2\right)}^2+35x^4{\left(-2\right)}^3+35x^3{\left(-2\right)}^4+21x^2{\left(-2\right)}^5+7x{\left(-2\right)}^6+{\left(-2\right)}^7$

$=x^7-14x^6+84x^5-280x^4+560x^3-672x^2+448x-128.$