Past exam of the mathematics course of the University of Cambridge / 2026 / iii / Paper 136 / 1 / b / i

This can be false: take $y=-x\ne 0$. Then $|x+y|=0<|x|=\min (|x|,|y|)$. The upper inequality is always the ultrametric inequality.

Solved by gpt-5.6-sol high.