Answer
$x^{40}$, $40x^{38},\ 780x^{36}$
Work Step by Step
$(a+b)^{n}=\left(\begin{array}{l}
n\\
0
\end{array}\right)a^{n}+\left(\begin{array}{l}
n\\
1
\end{array}\right)a^{n-1}b+\left(\begin{array}{l}
n\\
2
\end{array}\right)a^{n-2}b^{2}+\cdots+\left(\begin{array}{l}
n\\
n
\end{array}\right)b^{n}$
--------
The first three terms of $(x+\displaystyle \frac{1}{x})^{40}$ are
$\left(\begin{array}{l}
40\\
0
\end{array}\right)x^{40}=x^{40}$,
$\displaystyle \left(\begin{array}{l}
40\\
1
\end{array}\right)x^{39}(\frac{1}{x})^{1} =40x^{38}$, and
$\displaystyle \left(\begin{array}{l}
40\\
2
\end{array}\right)x^{38}(\frac{1}{x})^{2}= \frac{40\times 39\times x^{38}}{1\times 2\times x^{2}} =780x^{36}$.
