Trong các khẳng định sau, khẳng định nào đúng?
\(\left\{2\right\}\in\left\{\varnothing;\left\{1\right\};\left\{2\right\};\left\{1;2\right\}\right\}\)\(\left\{2\right\}\subset\left\{\varnothing;\left\{1\right\};\left\{2\right\};\left\{1;2\right\}\right\}\)\(\left\{1\right\}\subseteq\left\{\varnothing;\left\{1\right\};\left\{2\right\};\left\{1;2\right\}\right\}\)\(1\in\left\{\varnothing;\left\{1\right\};\left\{2\right\};\left\{1;2\right\}\right\}\)Hướng dẫn giải:
\(\left\{\varnothing;\left\{1\right\};\left\{2\right\};\left\{1;2\right\}\right\}\) gồm bốn phần tử \(\varnothing,\left\{1\right\},\left\{2\right\},\left\{1;2\right\}\) vì vậy khẳng định \(\left\{2\right\}\in\left\{\varnothing;\left\{1\right\};\left\{2\right\};\left\{1;2\right\}\right\}\) là khẳng định đúng.
