cos β =a/c
c=a/cos β
R=c/2=a/(2cos β)
H=R*tg α =(a/(2cos β ))*tg α
V=(1/3)*S_(осн)*H
S_(осн)=(1/2)a*c*sin β =(1/2)*a*(a/(2cos β ))=(1/4)a^2*(1/cos β )
V=(1/3)*(1/4)a^2*(1/cos β )*(a/(2cos β ))*tg α =(1/24)*a^3*tg α /cos^2 β