Comparing Areas - 3

Geometry Level 2

Consider a regular pentagon. Connect up the alternate sides to obtain a star.

Which is larger, the yellow area or the blue area?

Yellow area Blue area Equal

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1 solution

Ahmad Saad
Mar 11, 2016

Is there a way of comparing the areas without having to do all of that calculation? It's pretty clear that the blue area is much smaller than the yellow area, so at least the question is easy to answer.

I'm not sure what your calculations say. It seems like you want S 2 S 1 = 5 1 2 \frac{S_2} { S_1} = \frac{ \sqrt{ 5} - 1 } { 2 } , but in the next line you say that 2 S 2 + S 1 = 3 S 1 2 S_2 + S_1 = 3 S_ 1 implying that S 2 = S 1 S_2 = S_1 .

Chung Kevin - 5 years, 3 months ago

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Tanks for your remark. I've corrected my solution.

Ahmad Saad - 5 years, 3 months ago

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I believe that's correct. Thanks for fixing it. Great solution.

I didn't bother calculating the area, and I'm surprised that there's such a nice relationship there. I might post a problem about that.

Chung Kevin - 5 years, 3 months ago

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