If the volume of the solid obtained by rotating the region bounded by $\qquad f(x)=\cos x~~$ and the $~~x$-axis on the interval $~x\in\Big[-\dfrac\pi2,\dfrac\pi2\Big]$ about the line $~x=-\dfrac\pi2~$ can be written as $~a\pi^2\,$, what is $~a\,$? $\qquad a =~$[[☃ expression 1]]