So ab≤8,

And because a > 0 and b > 0, ab>0.

So 0

a+b = √( a+b)2 =√a2+B2+2ab =√ 16+2ab

Because 0

So 4 < √ 16+2ab≤4√2

All four.

√ indicates the root sign (this will do. I forgot to think about it. Sorry)