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Let-a-and-b-be-positive-real-numbers-with-a-3-b-3-a-b-and-k-a-2-4b-2-then-1-k-lt-1-2-k-gt-1-3-k-1-4-k-gt-2-




Question Number 21311 by Tinkutara last updated on 20/Sep/17
Let a and b be positive real numbers  with a^3  + b^3  = a − b, and k = a^2  + 4b^2 ,  then  (1) k < 1  (2) k >1  (3) k = 1  (4) k > 2
$$\mathrm{Let}\:{a}\:\mathrm{and}\:{b}\:\mathrm{be}\:\mathrm{positive}\:\mathrm{real}\:\mathrm{numbers} \\ $$$$\mathrm{with}\:{a}^{\mathrm{3}} \:+\:{b}^{\mathrm{3}} \:=\:{a}\:−\:{b},\:\mathrm{and}\:{k}\:=\:{a}^{\mathrm{2}} \:+\:\mathrm{4}{b}^{\mathrm{2}} , \\ $$$$\mathrm{then} \\ $$$$\left(\mathrm{1}\right)\:{k}\:<\:\mathrm{1} \\ $$$$\left(\mathrm{2}\right)\:{k}\:>\mathrm{1} \\ $$$$\left(\mathrm{3}\right)\:{k}\:=\:\mathrm{1} \\ $$$$\left(\mathrm{4}\right)\:{k}\:>\:\mathrm{2} \\ $$

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