求一个关于数学的英语笑话

1)定理/Theorem：1 = -1
证明/Proof：
x=1
x^2=x
x^2-1=x-1
(x+1)(x-1)=(x-1)
(x+1)=(x-1)/(x-1)
x+1=1
x=0
0=1
=> 0/0=1/1=1

2)Pi

Several scientists were all posed the following question: "What is pi ?"

The engineer said: "It is approximately 3 and 1/7"

The physicist said: "It is 3.14159"

The mathematician thought a bit, and replied "It is equal to pi".

A nutritionist: "Pie is a healthy and delicious dessert!"

3)
A mathematician, a physicist, and an engineer were traveling through Scotland when they saw a black sheep through the window of the train.

"Aha," says the engineer, "I see that Scottish sheep are black."

"Hmm," says the physicist, "You mean that some Scottish sheep are black."

"No," says the mathematician, "All we kno