Bài làm4) Chứng minh rằng:
$1 - 10C_{2n}^1 + {10^2}C_{2n}^2 - {10^3}C_{2n}^3 + ... - {10^{2n - 1}}C_{2n}^{2n - 1} + {10^{2n}} = {81^n}$
Ta có:\[{(1 - x)^{2n}} = C_{2n}^0 - xC_{2n}^1 + {x^2}C_{2n}^2 - ... - {x^{2x - 1}}.C_{2n}^{2n - 1} + {x^{2n}}C_{2n}^{2n}\]
Thay $x=10$ ta thu được:
\[{(1 - 10)^{2n}} = 1 - 10C_{2n}^1 + {10^2}C_{2n}^2 - ... - {10^{2x - 1}}.C_{2n}^{2n - 1} + {10^{2n}}C_{2n}^{2n}\]
\[ \Leftrightarrow {( - 9)^{2n}} = 1 - 10C_{2n}^1 + {10^2}C_{2n}^2 - ... - {10^{2x - 1}}.C_{2n}^{2n - 1} + {10^{2n}}C_{2n}^{2n}\]$ \Leftrightarrow {81^n} = 1 - 10C_{2n}^1 + {10^2}C_{2n}^2 - ... - {10^{2x - 1}}.C_{2n}^{2n - 1} + {10^{2n}}C_{2n}^{2n}$ (đpcm)