<SHO=a,<AOB=b,OA=OB=R,SH_|_AB,OH_|_AB
AB=2AH=2*AO*sin<AOH=2Rsin(b/2)
OH=AB*cos<AOB=Rcos(b/2)
SH=OH/cos<SHO=Rcos(b/2)/cosa
SO=OH*tg<SHO=Rcos)b/2)tga
SB=√(OB²+SO²)=√(R²+R²cos²(b/2)tg²a)=R√√(1+cos²(b/2)tg²a)
Sсеч=1/2*AB*SH
S=1/2*2R*sin(b/2)*Rcos(b/2)/2cosa=R²sinb/2cosa
чертеж во вложении
5*6*4см^2 - 2*4*4см^2 = 120см^2 - 32см^2 = 88см^2
Ответ: 88см^2