Let −S
S
be the set {−s:s∈S}
s
s
S
where −S
S
is the set that contains negatives of the members of S
S
. We want to prove that inf(S)=−sup(−S)
S
S
Here is how I proved it Let s0=sup(−S)
s
0
S
. That is for all −s1∈−S
s
1
S
then −s1≤s0
s
1
s
0
. Multiplying both sides by −1
1
we get −s0≤s1
s
0
s
1
for all s1∈S
s
1
S
. So inf(S)=−s0=−sup(−S)
S
s
0
S
It looks short and sweet. Not sure if its right though.