# Formulae 公式

Refer to the last chapter for symbol representation.

Square and rhombus 正方形和菱形: $4s$
Rectangle and parallelogram 长方形和平行四边形: $2(a+b)$ ($a$ and $b$ refer to the side length of two adjacent sides) （$a$$b$ 分别指两相邻边的长度）

## ellipticals 椭圆形

Call a circle’s circumference (perimeter) $C$ and diameter $d$. Then $\pi =\dfrac {C}{d}$, so $C=2\pi r=\pi d$.

# Techniques 技巧

When we meet irregular shapes, we can ‘move’ their perimeter without changing it’s value, as below.

Note by Jeff Giff
1 month, 3 weeks ago

This discussion board is a place to discuss our Daily Challenges and the math and science related to those challenges. Explanations are more than just a solution — they should explain the steps and thinking strategies that you used to obtain the solution. Comments should further the discussion of math and science.

When posting on Brilliant:

• Use the emojis to react to an explanation, whether you're congratulating a job well done , or just really confused .
• Ask specific questions about the challenge or the steps in somebody's explanation. Well-posed questions can add a lot to the discussion, but posting "I don't understand!" doesn't help anyone.
• Try to contribute something new to the discussion, whether it is an extension, generalization or other idea related to the challenge.

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Note: you must add a full line of space before and after lists for them to show up correctly
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a_{i-1} $a_{i-1}$
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\sin \theta $\sin \theta$
\boxed{123} $\boxed{123}$

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Jeff, this is good, but I don't understand Chinese.

Is there any other languages apart from Chinese and English that you can speak?

- 1 month, 3 weeks ago

Nope.

- 1 month, 3 weeks ago

Sorry :/

- 1 month, 3 weeks ago

That's okay.

- 1 month, 3 weeks ago