\(a^2+5 \vdots a^2+2\\\to a^2+2+3 \vdots a^2+2\\\to 3 \vdots a^2+2\\to a^2+2 \in Ư(3)={1,-1,3,-3}\\ VÌ \,\, a^2+2 \geq 2\\\to a^2+2=3\\\to a^2=1\\\to \left[ \begin{array}{l}a=1\\a=-1\end{array} \right.\\\text{Vậy a=1 hoặc a=-1}\\\text{@Water}\)