Respostas

2014-05-10T14:28:23-03:00
Lembrando do termo geral para o desenvolvimento

(x+y)^n

\boxed{T_{k+1}=\binom{n}{k}x^{n-k}y^k}

Separando as informações .

20-k=17\\k=3

n=20

x=a

y=-1

Aplicando no termo geral

T_{16}=\binom{20}{3}a^{20-3}*(-1)^3\\
\\T_{16}=\binom{20}{3}a^{17}*(-1)^3

\boxed{T_{16}=-1140a^{17}}

\boxed{\boxed{\therefore ~~-1140}}

Caso alguém questione porque de o k ser 3

\boxed{\binom{20}{0}a^{20}~~\binom{20}{1}a^{19}~~\binom{20}{2}a^{18}~~\binom{20}{3}a^{17}}

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