The measure of angle GFD of the circumscribed circle is; A: 40°
From the figure, we can apply the arc angles summation formula to get;
Major angle ∠FED + Minor angle ∠FED = 360°
We are given that Major arc FD measures 280°. Thus;
280° + Minor angle ∠FED = 360°
Minor angle ∠FED = 360° - 280°
Minor angle ∠FED = 80°
Also, we know that;
∠FED + ∠FGD = 180°
Thus, putting ∠FED = 80° gives us;
80° + ∠FGD = 180°
Subtract 80° from both sides using subtraction property of equality to get;
∠FGD = 180° - 80°
∠FGD = 100°
Now, GF and GD are the tangents to the circle from the same point G. Thus, we can say that;
GD = GF
Therefore,
∠FDG = ∠GFD = x
(This is because ∠FDG and ∠GFD are the angles opposite to equal sides.
In triangle FGD, we have sum of interior angles = 180°
Therefore, we have the expression;
∠FDG + ∠FGD + ∠GFD = 180° (due to the fact that sum of angles in a triangle is equal to 180°)
x + 100° + x = 180°
2x = 180° - 100°
2x = 80°
x = 40°
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