This is an excellent quick revision of the French Baccalaureat Math during the first month of French university. (Unfortunately common A-level Math syllabus lacks such rigourous Math foundation.)

Most non-rigourous high-school students / teachers abuse the use of :

“=> ” , “<=>” .

**Prove by “Reductio par Absudum” 反证法 (by Contradiction) is a clever mathematical logic :**

**$latex boxed {(A => B) <=> (non B => non A)} &s=3$**

**Famous Examples: 1) Prove $latex sqrt 2 $ is irrational ; 2) There are infinite prime numbers (both by Greek mathematician Euclid 3,000 years ago)**

The young teacher showed the techniques of proving Mapping:

**Surjective** (On-to) – best understood in Chinese 满射 (Full Mapping)

**Injective** (1-to-1) 单射

**Bijective** (On-to & 1-to-1) 双射

He used an analogy of (the Set of) red Indians shooting (the Set of bisons 野牛):

All bisons are shot by arrows from* 1 or more *Indians. (Surjective…

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