[Toán 11] Tổ hợp xác suất

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ngoc1thu2

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mu_vodoi

Ta có : a0=1 và |ak|= $3^k. C_{20}^k$


$Voi \ \ k>0 :\ \ (k+1) |a_k| = ( a_0 + a_1 + a_2 +...+ a_20) + (a_1 + 2 a_2 + ...+ 20a_{20} ) = (1+3)^{20} + 3.20(C_{19}^0+3 C_{19}^1+....+3^{19}.C_{19}^{19}) = 4^{20} + 60. (1+3)^{19} = 16.4^{20} = 4^{22}$


$S = 4^{22}$
 
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