The reverse triangle inequality $$|x-y|\ge ||x|-|y||$$ is a classic textbook exercise using the ordinary triangle inequality, but it is also a useful result in calculus or mathematical analysis. We prove it here using two different methods.
The reverse triangle inequality $$|x-y|\ge ||x|-|y||$$ is a classic textbook exercise using the ordinary triangle inequality, but it is also a useful result in calculus or mathematical analysis. We prove it here using two different methods.