1) A=g*h/2
2) sin (alpha/2)=g/(2a)
g=2a*sin (alpha/2)
3) cos (alpha/2)=h/a
h=cos (alpha/2)*a
In 1)
A=2a*sin(alpha/2)*a*cos (alpha/2)/2
=a^2*sin (alpha/2)*cos (alpha/2)
4) A'=a^2*(1/2*cos^2 (alpha/2)-1/2*sin^2 (alpha/2))=0
sin (alpha/2)=cos (alpha/2)
sin (alpha/2)/cos (alpha/2)=1
Tan (alpha/2)=1
alpha=2*arctan (1)
=90°