Viết khai triển theo nhị thức Newton: a) (x + y)6; b) (1 – 2x)5.

a) (x + y)⁶ $=C_6^0{x}^6+C_6^1{x}^{5}y+C_6^2{x}^4{y}^2+C_6^3{x}^3{y}^3+C_6^4{x}^2{y}^4+C_6^{5}x{y}^5+C_6^6{y}^6$

$=x^6+C_6^1{x}^{5}y+C_6^2{x}^4{y}^2+C_6^3{x}^3{y}^3+C_6^4{x}^2{y}^4+C_6^{5}x{y}^5+y^6.$

b) (1 – 2x)⁵ = [(–2x) + 1]⁵

= $C_5^0$(–2x)⁵ + $C_5^1$(–2x)⁴1 + $C_5^2$(–2x)³1² + $C_5^3$(–2x)²1³ + $C_5^4$(–2x)1⁴ + $C_5^5$1⁵

= –2⁵x⁵ + $C_5^1$2⁴x⁴ – $C_5^2$2³x³ + $C_5^3$2²x¹ + $C_5^4$2x + 1.