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Rational numbers are horrible for most numerical computing. The denominators double in length with every multiplication, so computation speed gets slow exponentially.

Any time there’s any degree of uncertainty about a quantity (e.g. it comes from a physical measurement) there’s also no longer any advantage to using rational arithmetic. This turns out to encompass most practical situations.

Rational arithmetic also breaks down entirely in the face of square roots or trig functions, unless you go for a fully symbolic computation environment, which gets even much slower.

Rational arithmetic is mostly nice when the problems have been carefully chosen so the operations will stay rational and the answers will work out nicely, e.g. in high school homework.




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