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Define the generating functions $\text{B}(x)=\displaystyle{} \sum_{n=0}^{\infty} 2^{n} x^{n}$ and $F(x)=\displaystyle{} \sum_{n=0}^{\infty} f_{n} x^{n}$ where $f_{n}$ is the Fibonacci sequence determined by the recurrence relation
$$
\begin{aligned}
f_{0} &=0 \quad \text { and } \quad f_{1}=1 \\
f_{n} &=f_{n-1}+f_{n-2} & \text { for } n \geq 2\\
\end{aligned}
$$
Let $\text{G}(x)=\text{B}(x) \times \text{F}(x)$.

What is the coefficient of $x^{5}$ is $\mathrm{G}(x)?$
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$$
\begin{aligned}
&\text{G}(x)=\text{B}(x) \times \text{F}(x) \\
&=\left(2^{0} x^{0}+2^{1} x^{1}+2^{2} x^{2}+2^{3} x^{3}+2^{4} x^{4}+2^{5} x^{5}+2^{6} x^{6}+\ldots\right) \times \\
&\left(1 x^{1}+1 x^{2}+2 x^{3}+3 x^{4}+5 x^{5}+8 x^{6}+\ldots\right)
\end{aligned}
$$
So, Coefficient of $x^{5}$ will be$:5+6+8+8+16=43.$

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