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**emakarov** Summation #1: $\displaystyle\sum_{k=1}^n\frac{k}{(k+1)!}$. Try the second one.

5. $\Bbb Q\cap \Bbb R=\Bbb Q$. The symbol $\cap$ means intersection. For any two sets $A$ and $B$, the set $A\cap B$ contains elements that belong to both $A$ and $B$. So, $\Bbb Q\cap \Bbb R$ contains numbers that are both in $\Bbb Q$ and in $\Bbb R$. But if a number is in $\Bbb Q$, it's also in $\Bbb R$, so $\Bbb Q\cap \Bbb R$ contains the same elements as simply $\Bbb Q$. The statement is true.

6. $\Bbb Q\cup\Bbb Z=\Bbb Q$. The symbol $\cup$ means union. For any two sets $A$ and $B$, the set $A\cup B$ contains elements that belong to $A$, to $B$ or both. But $\Bbb Z$ is a subset of $\Bbb Q$, so adding it to $\Bbb Q$ does not make the latter set larger. The statement is true.

Try questions 7 and 8.