A regular five star is inscribed in a circle of radius 10
If A denote the area of shaded region
Find 8 . 8 4 1 A
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Sorry for that , can you change the answer. upvoted
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I have updated the answer to 992.464
@megh choksi You should clarify what a "regular five star" is. It is not immediately obvious that the 2 edges lie on the same line.
By sine law, we have
sin 3 6 x = sin 1 2 6 1 0
x ≈ 7 . 2 6 5 4
A = 1 0 ( 2 1 ) ( x ) ( 1 0 ) ( sin 1 8 ) = 5 ( 7 . 2 6 5 4 ) ( 1 0 ) ( sin 1 8 ) ≈ 1 1 2 . 2 5 7
The desired answer is 8 . 8 4 1 ( 1 1 2 . 2 5 7 ) = 9 9 2 . 4 6 4
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Let A be the top vertex of the star, O be the center of the star, (and circle), and B be the vertex of the star immediately below and to the right of A . Thus A O = 1 0 .
Since the interior angle of each of the 'points' of the star is one-half the central angle of a pentagon, we have that ∠ O A B = ( 2 1 ) ( 2 1 ) 7 2 ∘ = 1 8 ∘ .
Now the star is composed of 1 0 triangles congruent to Δ A O B , and thus ∠ A O B = ( 1 0 1 ) 3 6 0 ∘ = 3 6 ∘ .
This means that ∠ A B O = 1 2 6 ∘ . Now use the Sine Law on Δ A O B to find that A B = A O ∗ sin ( 1 2 6 ∘ ) sin ( 3 6 ∘ ) = 1 0 ∗ tan ( 3 6 ∘ ) .
Thus Δ A O B has a base length of 1 0 and a height of 1 0 ∗ tan ( 3 6 ∘ ) sin ( 1 8 ∘ ) , and so the area of the star is
A = 1 0 ∗ ( 2 1 ) ∗ 1 0 ∗ 1 0 ∗ tan ( 3 6 ∘ ) sin ( 1 8 ∘ ) = 5 0 0 ∗ tan ( 3 6 ∘ ) sin ( 1 8 ∘ ) .
The solution is then 8 . 8 4 1 ∗ A = 9 9 2 . 4 6 4 to 3 decimal places.
(This differs slightly from the posted answer of 9 9 2 . 5 2 4 , (probably due to rounding), so I'm guessing that any answer that rounds to 9 9 2 . 5 will be considered correct.)