`x^2 -4x-5=0`

`<=>x^2 -5x+x-5=0`

`<=>x(x-5)+(x-5)=0`

`<=>(x+1)(x-5)=0`

`<=>`\(\left[ \begin{array}{l}x=-1\\x=5\end{array} \right.\) 

Mà `x in N =>A={5}`

`(x-1)(x^2 -4)=0`

`<=>`\(\left[ \begin{array}{l}x-1=0\\x^2-4=0\end{array} \right.\) 

`<=>`\(\left[ \begin{array}{l}x=1\\x^2=4\end{array} \right.\) 

`<=>`\(\left[ \begin{array}{l}x=1\\x=2\\x=-2\end{array} \right.\) 

Vì `x in R =>B={-2;1;2}`

`=>A∪B={-2;1;2;5}`