Since ABDC quadrangular The interior angles of a quadrilateral is 360 degrees, so that hat {A} + hat {B} + hat {D} + hat {C} = 360. Instead, the size of a tight corner to be 85 + 20 + 360 = 360-4x + x = x + 360-85-20-360 -4x -3x = -105 x = frac {-105} {- 3}. x = 35 , so the ACD = 35 degrees.
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