R=H/2+L^2/(8*H)
=3/2+11.7^2/(8*3)
=7.204m
A=2*ARC SIN((L/2)/R)
=2*ARC SIN((11.7/2)/7.204)
=108.6度
S=PI*R^2*A/360-L*R*COS(A/2)/2
=PI*7.204^2*108.6/360-11.7*7.204*COS(108.6/2)/2
=24.588平方米