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ABC is an isosceles triangle. BDE, EFG, GHI and IJC are four equal isosceles triangles inside ABC triangle. D and F,F and H,H and J are connected by circular arcs. The angle ABC is 30 degrees and BE is 1m. What is the area of the shaded region ?
Since $$\angle ABC=30^o$$ and $$\triangle DBE$$ is an isosceles triangle, $$\angle BDE=120^o$$
Applying the cosine rule in $$\triangle BDE$$,
$$BE^2=BD^2+DE^2-2\times BD\times DEcos(120)$$
$$1=2BD^2+BD^2$$ [since $$BD=DE$$]
$$BD=\frac{1}{\sqrt 3}$$
Thus, $$BD=DE=EF=FG=GH=HI=IJ=JC=\frac{1}{\sqrt 3}$$
Area of $$\triangle BDE=\frac{1}{2}\times BD\times DE\times sin(120)=\frac{1}{2}\times \frac{1}{\sqrt 3}\times \frac{1}{\sqrt 3}\times \frac{\sqrt 3}{2}=\frac{1}{4\sqrt 3}$$
The total area of all smaller isosceles triangles = $$4\times \frac{1}{4\sqrt 3}=\frac{1}{\sqrt 3} m^2$$
$$\angle DEF=\angle FGH = \angle HIJ=120^o$$
Thus, the total area of circular arcs = $$\pi\times (\frac{1}{\sqrt 3})^2=\frac{\pi}{3} m^2$$
Now, side $$BC=BE+EG+GI+IC=4 m$$
Using the cosine rule in $$\triangle ABC$$, we get $$AB=AC=\frac{4}{\sqrt 3}$$
Thus, Area of $$\triangle ABC=\frac{1}{2}\times \frac{4}{\sqrt 3}\times \frac{4}{\sqrt 3}\times sin(120)=\frac{4}{\sqrt 3} m^2$$
Thus, the area of the shaded region = Area of $$\triangle ABC$$ - Area of smaller isosceles triangles - Area of the circular sections
=$$\frac{4}{\sqrt 3}-\frac{1}{\sqrt 3}-\frac{\pi}{3}=\sqrt 3 - \frac{\pi}{3}$$
Hence, the answer is option C.
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