Answer
$$\frac{2}{3}(e^t+1)^{3/2}+c
$$
Work Step by Step
Let $u=e^t+1$, then $du=e^t dt$. Now, we have
$$
\int e^{t} \sqrt{e^{t}+1} d t=\int u^{1/2}du\\
=\frac{1}{3/2} u^{3/2}+c=\frac{2}{3}(e^t+1)^{3/2}+c
$$
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