Lời giải:

$x\in \mathbb{Z}^+, y\in\mathbb{Z}^-$ thì $xy\in\mathbb{Z}^-$

Còn $x+y$ không xác định được là nguyên âm hay nguyên dương vì nó có thể xảy ra cả 2 TH:

x=5, y=-3 thì x+y nguyên dương 

x=1, y=-3 thì x+y nguyên âm. 

In recent years, the increasing celebration of festivals such as Christmas and Halloween among young Vietnamese people has sparked a debate on cultural influence. While some view this trend as harmless, others express concern about the potential erosion of traditional values. 

In my opinion, celebrating these festivals can be beneficial for several reasons. Firstly, it promotes cultural exchange and broadens horizons. Embracing different traditions allows young people to become more open-minded and globally aware. This can foster a more inclusive society where diversity is appreciated rather than feared. 

Secondly, these celebrations provide opportunities for socializing and community building. Festivals like Christmas and Halloween bring people together, creating joyful and memorable experiences. They offer a break from routine and can enhance the sense of belonging among young people, especially in urban areas where traditional community ties might be weaker.

However, it is crucial to balance this with the preservation of Vietnamese traditions. Promoting traditional festivals and cultural education can ensure that young people remain connected to their roots while enjoying the benefits of global cultural exposure. In conclusion, the celebration of foreign festivals should be seen as an enriching addition to, rather than a replacement for, Vietnamese culture.

\(A'A\perp\left(ABCD\right)\Rightarrow\widehat{A'BA}\) là góc giữa A'B và (ABCD)

\(tan\widehat{A'BA}=\dfrac{A'A}{AB}=\sqrt{3}\)

\(\Rightarrow\widehat{A'BA}=60^0\)