Question

CMR: \(\dfrac{1}{\left(1+a\right)^2}+\dfrac{1}{\left(1+b\right)^2}\ge\dfrac{1}{1+ab}\forall a,b\ge0\)

 

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Answer 1

=>(ab-1)^2+ab(a-b)^2>=0

=>a^2b^2-2ab+1+ab(a^2-2ab+b^2)>=0

=>a^2b^2-2ab+1+a^3b-2a^2b^2+ab^3>=0

=>a^3b+ab^3-a^2b^2-2ab+1>=0

=>ab(a^2+b^2)-2ab-a^2b^2+1>=0

=>ab(a^2+b^2-2-ab)+1>=0(luôn đúng)