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It is the e mathematical constant, Euler's constant.
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Pi is a mathematical constant because it is the same for every perfect Circle.
Pi = Circumference/Diameter of any Circle.
It is constant because no matter the size of the circle that ratio always comes out to Pi.
Hope they helped :)
A lot of mathematical problems about travelling distances, assume that you are travelling at a constant speed.
So as not to confuse a mathematical constant with a pastry.