Allgemein
c^2 = a^2 + b^2
A23 = 1/2·a·b + 1/2·pi·(b/2)^2 + 1/2·pi·(a/2)^2 - 1/2·pi·(c/2)^2
A23 = (pi·a^2 + 4·a·b + pi·b^2 - pi·c^2)/8
A23 = (pi·a^2 + 4·a·b + pi·b^2 - pi·(a^2 + b^2))/8
A23 = 1/2·a·b
Bei dir gilt nun
85^2 = a^2 + 36^2 --> a = 77 cm
A23 = 1/2·77·36 = 1386 cm²