实数有两种构成方法：
实数可以分为有理数（如 42、-23/129）和无理数（如 π、根号2），或者代数数和超越数（有理数都是代数数）两类
实数和代数数的关系：
代数数不一定是实数，实数也不一定是代数数。
所以实数轴上的是（实代数数）和（超越数）
Algebraic number:
An algebraic number is any complex number (including real numbers) that is a root of a non-zero polynomial (that is, a value which causes the polynomial to equal 0) in one variable with rational coefficients (or equivalently – by clearing denominators – with integer coefficients). All integers and rational numbers are algebraic, as are all roots of integers. The same is not true for all real numbers or all complex numbers. Those real and complex numbers which are not algebraic are called transcendental numbers. They include π and e. Almost all complex numbers are transcendental.
Transcendental number:
In mathematics, a transcendental number is a real or complex number that is not algebraic—that is, it is not a root of a nonzero polynomial equation with integer (or, equivalently, rational) coefficients. The best-known transcendental numbers are π and e.
超越数介绍：
https://www.youtube.com/watch?v=seUU2bZtfgM
https://www.youtube.com/watch?v=3xyYs_eQTUc
e 和 π 是超越数的一个证明：
http://sixthform.info/maths/files/pitrans.pdf
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