Geometry

Geometry Level 4

A regular five star is inscribed in a circle of radius 10

If A denote the area of shaded region

Find 8.841 A 8.841A


The answer is 992.464.

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2 solutions

Let A A be the top vertex of the star, O O be the center of the star, (and circle), and B B be the vertex of the star immediately below and to the right of A A . Thus A O = 10 AO = 10 .

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 = ( 1 2 ) ( 1 2 ) 7 2 = 1 8 \angle OAB = (\frac{1}{2})(\frac{1}{2}) 72^{\circ} = 18^{\circ} .

Now the star is composed of 10 10 triangles congruent to Δ A O B \Delta AOB , and thus A O B = ( 1 10 ) 36 0 = 3 6 \angle AOB = (\frac{1}{10}) 360^{\circ} = 36^{\circ} .

This means that A B O = 12 6 \angle ABO = 126^{\circ} . Now use the Sine Law on Δ A O B \Delta AOB to find that A B = A O sin ( 3 6 ) sin ( 12 6 ) = 10 tan ( 3 6 ) AB = AO * \dfrac{\sin(36^{\circ})}{\sin(126^{\circ})} = 10*\tan(36^{\circ}) .

Thus Δ A O B \Delta AOB has a base length of 10 10 and a height of 10 tan ( 3 6 ) sin ( 1 8 ) 10*\tan(36^{\circ})\sin(18^{\circ}) , and so the area of the star is

A = 10 ( 1 2 ) 10 10 tan ( 3 6 ) sin ( 1 8 ) = 500 tan ( 3 6 ) sin ( 1 8 ) A = 10*(\frac{1}{2})*10*10*\tan(36^{\circ})\sin(18^{\circ}) = 500*\tan(36^{\circ})\sin(18^{\circ}) .

The solution is then 8.841 A = 992.464 8.841*A = \boxed{992.464} to 3 3 decimal places.

(This differs slightly from the posted answer of 992.524 992.524 , (probably due to rounding), so I'm guessing that any answer that rounds to 992.5 992.5 will be considered correct.)

Sorry for that , can you change the answer. upvoted

U Z - 6 years, 6 months ago

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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.

Calvin Lin Staff - 6 years, 6 months ago

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i think it is absolutely correct sir

sudoku subbu - 6 years, 5 months ago

By sine law, we have

x sin 36 = 10 sin 126 \dfrac{x}{\sin 36}=\dfrac{10}{\sin 126}

x 7.2654 x \approx 7.2654

A = 10 ( 1 2 ) ( x ) ( 10 ) ( sin 18 ) = 5 ( 7.2654 ) ( 10 ) ( sin 18 ) 112.257 A=10\left(\dfrac{1}{2}\right)(x)(10)(\sin 18)=5(7.2654)(10)(\sin 18) \approx112.257

The desired answer is 8.841 ( 112.257 ) = 8.841(112.257)= 992.464 \boxed{992.464}

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