In: Math
If a² + b² + c² = 1, then ab + bc + ac lies in which interval
Given that a² + b² + c² = 1 …(i)
We know (a+b+c)² ≥ 0
a²+b²+c²+2ab+2bc+2ac ≥ 0
=> 1 + 2(ab+bc+ac)≥ 0 (from (i))
=> 2(ab+bc+ac)≥ -1
=> (ab+bc+ac)≥ -½ …(ii)
We know that ½ [(a-b)² + (b-c)² + (c-a)²]≥ 0
=> a² + b² + c² – ab – bc – ac ≥ 0
=> ab + bc + ac ≤ 1 …(iii)
Combining (ii) and (iii)
-½ ≤ (ab+bc+ac) ≤ 1
Therefore (ab+bc+ac) ∈ [-½, 1]
(ab+bc+ac) ∈ [-½, 1]