I'm assuming that you're saying that one side of the quadrilateral is the same as the circle's diameter, and we have two adjacent sides to this that are 8 and 12m, respectively. Then the circle's radius = 10 m
To find the remaining side, we need to first find the central angles intercepting the 8 and 12 m sides.....
We can use the Law of Cosines in both cases....for the 12m side, we have
12^2 = 10^2 + 10^2 - 2(10)(10)cosΘ ......simplifying, we have
cosΘ= 7/25 = cos-1(7/25) = about 73.739795291688°
For the 8m side, we have
8^2 = 10^2 + 10^2 - 2(10(10)cosΘ ...simplifying again, we have
cosΘ = 17/25 = cos-1(17/25) = about 47.156356956404°
So, since the diameter would span 180° of arc, the remaing side must span (180 - 73.739795291688 - 47.156356956404)° = 59.103847751908° of arc
So...the remaining side... (s).... is given by
s^2 = 10^2 + 10^2 - 2(10)(10)cos(59.103847751908)
s = √[10^2 + 10^2 - 2(10)(10)cos(59.103847751908)] = about 9.86 m
This makes sense....the greater side intercepts the greatest arc, the next greatest side intercepts the next greatest arc, etc.
