There are two versions of this question. I edited the question above changing is the

shaded region that I suppose is the area inside the circle but OUTSIDE the two triangles.

Now, the radius is 10 and the area is 100 \pi. The area of the shadow region is \(\frac{80}{100} = \frac{4}{5}\) of the circle and the rest is \(\frac{1}{5}\) of the circle.

We do know that the central angle in a circle is always \(360\) and \(\frac{1}{5}\) of 360 is 72°.

\(a + b + c + d + 72 = 360\)

\(a + b + c + d = 288\)

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