



 
Hi Aishwarya, Here is the definition of the least upper bound from MathWorld.
Suppose $a$ and $b$ are least upper bounds (lub) of a set $S$ in the real line. By 1. above both $a$ and $b$ are upper bounds of $S.$ Since $a$ is a lub of $S$ what does 2. tell you about the relationship between $a$ and $b?$ Since $b$ is a lub of $S$ what does 2. tell you about the relationship between $b$ and $a?$ Penny  


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