Cute Little Shuriken

Geometry Level 4

Find the area of the region colored in orange.

For your final step, use the approximation π = 22 7 \pi = \dfrac{22}7 .

Give your answer to 1 decimal place.


The answer is 15.5.

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3 solutions

Jason Chrysoprase
May 17, 2016

Assume that n = Circle segment area and x + y + n = Circle sector area \Large \text{Assume that }n = \text{Circle segment area and}\space x+y+n = \text{Circle sector area}

x + y = Sector Area Segment Area \Large x + y = \text{ Sector Area }- \text{Segment Area}

= ( x + y + n ) n = 30 360 × π × 7 2 ( 60 360 × π × 7 2 3 4 × 7 2 ) = 49 12 × 22 7 ( 49 6 × 22 7 49 4 × 3 ) = 77 6 77 3 + 49 4 3 \Large = ( x + y + n ) - n \\ \Large = \frac {30}{360} \times \pi \times 7^2 - ( \frac { 60}{360} \times \pi \times 7^2 - \frac {\sqrt{3}}{4} \times 7^2) \\ \Large = \frac{ 49}{12} \times \frac{22}{7} - ( \frac{49}{6} \times \frac{22}{7} - \frac{49}{4} \times \sqrt{3}) \\ \Large = \frac{77}{6} - \frac{77}{3} + \frac{49}{4} \sqrt{3}

Shaded Area = Square Area 4 ( x + y ) = 7 2 4 ( 77 6 77 3 + 49 4 3 ) = 15.462... 15.5 \Large \text{Shaded Area} = \text{ Square Area} - 4 ( x+y) \\ \Large = 7^2 - 4(\frac{77}{6} - \frac{77}{3} + \frac{49}{4} \sqrt{3}) \\ \Large = 15.462... \\ \Large \approx 15.5

Exactly the same as this question , I just took my answer from here and substituted the side of the square to get my answer

Hung Woei Neoh - 5 years ago

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Howwww ????

I didn't even know about that question

Jason Chrysoprase - 5 years ago

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Popped up in my problems of the day

Hung Woei Neoh - 5 years ago

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@Hung Woei Neoh LOL, that's just coincidence

Jason Chrysoprase - 5 years ago

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@Jason Chrysoprase Exactly. Anyway, I solved it the same way as you XD

Hung Woei Neoh - 5 years ago

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@Hung Woei Neoh Which one solution is easier to you, mine or the others ?

Jason Chrysoprase - 5 years ago

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@Jason Chrysoprase I dunno. It depends on how you see the shapes and what theorems or formulas you know

Hung Woei Neoh - 5 years ago

ABCD is a square with sides a. O is the center. It is divided ito 4 equal squares. A quarter of the flower is shown as area OMN. L e t M R C D . I n r t . Δ M C R , 2 R M = H y p . M C . M C R = 3 0 o o f s y m m e t r y , B C N = 3 0 o . C l e a r l y N C M = 3 0 o . Δ M N C i s a 3 0 o a . C R = 3 2 a , M O = 3 2 a 1 2 a . \text{ABCD is a square with sides a. O is the center. It is divided ito 4 equal squares.}\\ \text{A quarter of the flower is shown as area OMN.}\\ Let~ MR \bot CD. ~~~In~ rt. ~\Delta ~MCR, ~~2RM=Hyp.~MC. ~~\implies ~\angle MCR=30^o\\ \because ~of~ symmetry, ~\angle BCN=30^o. ~Clearly~\angle NCM=30^o.\\ \therefore ~\Delta ~ MNC ~is ~a-30^o-a.~~~~~~CR=\dfrac{\sqrt3} 2 a, ~~~\therefore ~\color{#3D99F6}{MO=\dfrac{\sqrt3} 2 a-\dfrac 1 2 a.}\\ R e q u i r e d a r e a = 4 { a r e a C M N } = 4 { a r e a o f s e c t o r N C M ( a r e a s o f Δ s M O C a n d N O C ) } = 4 { π a 2 3 0 o 36 0 o 2 1 2 M O a 2 = a 2 3 ( π + 3 3 3 . ) a = 7 a n d π = 22 / 7 , s u b s t i t u t i n g w e g e t a r e a = 15.4628. This is the copy of my solution of the problem 4 months ago. Required ~area=4*\{area ~ CMN\}\\ =4*\{area ~ of ~sector ~ NCM- (areas~ of ~\Delta s~ MOC ~ and ~NOC )\}\\ =4*\{\pi*a^2*\dfrac{30^o}{360^o}- 2*\frac 1 2 *MO*\dfrac a 2\\ =\dfrac{ a^2} 3 (\pi +3-3\sqrt3. )\\ a=7\ and\ \pi=22/7, \ substituting \ we\ get\ area=15.4628. \\ \text{This is the copy of my solution of the problem 4 months ago.}

Hana Wehbi
May 17, 2016

The area of the shaded region is: a 2 3 \frac{a^{2}}{3} ( 22 / 7 + 3 3 3 *(22/7+3-3\sqrt{3} ), where a = 7 a=7 . We can draw a square inside the orange area whose side is equal to a 2 3 a\sqrt{2-\sqrt{3}} by using Pythagoras's theorem. Hence, the area of the square is a 2 ( 2 3 ) a^{2}(2-\sqrt{3}) . Also, the area between our square and the orange area is: 4 ( t h e a r e a o f t h e f o u r c i r c u l a r s e c t o r s 4(the\ area\ of\ the four\ circular\ sectors = 4 ( a 2 12 4(\frac{a^{2}}{12} ) ( 22 7 (\frac{22}{7} -3)).

Thus, total area of the orange part: 7 2 ( 2 3 ) 7^{2}(2-\sqrt{3}) + 4 ( 7 2 12 4(\frac{7^{2}}{12} ) ( 22 7 (\frac{22}{7} - 3) = 7 2 3 \frac{7^{2}}{3} ( 22 / 7 + 3 3 3 (22/7+3-3\sqrt{3} )= 15.5

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