theorem :: FUZZY_7:50

for a1, c, a2, d being Real st c > 0 & d > 0 & a1 < a2 holds
centroid ((d (#) (TrapezoidalFS ((a1 - c),a1,a2,(a2 + c)))),['(a1 - c),(a2 + c)']) = (a1 + a2) / 2