+(a_2 \times a_3) (a_1((a_3 \times a_1) a_2))
+\]
+\[
+\textrm{Hint 2: }
+(a_2 \times a_3) (a_1((a_3 \times a_1) a_2)) =
+(a_2 \times a_3) (a_1((a_2 \times a_3) a_1)) =
+(a_1 (a_2 \times a_3))^2
+\]
+\[
+\Rightarrow V_{rec}=\frac{(2\pi)^3}{a_1(a_2 \times a_3)}=
+\frac{(2\pi)^3}{V_{real}}