10^-6n^2 and n^2 both are O(n^2) how?

general
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1

if we multiply very very small number in n^2 then big O of n^2 and (very very small)*n^2 is n^2,why and how?

2

That’s literally how big O notation is defined. Any constant does not matter for the asymptotic behavior. Wikipedia defines the term pretty well: Big O notation - Wikipedia

3

big O is more of “how much slower code gets with larger input” rather than just how slow it is.