ANSWER
[tex]\mleft\lbrace1,2,3,4,5\mright\rbrace[/tex]
EXPLANATION
We want to find the union of the two sets given:
[tex]\begin{gathered} A=\mleft\lbrace2,5\mright\rbrace \\ B=\mleft\lbrace1,2,3,4,5\mright\rbrace \end{gathered}[/tex]
The union of the two sets is the combined set of all numbers that are in the two sets.
Therefore, we have that:
[tex]A\cup B=\mleft\lbrace1,2,3,4,5\mright\rbrace[/tex]
That is the answer.
