Subtracting Cube Roots

Algebra Level 3

6 3 + 10 3 6 3 10 3 \sqrt[3]{6\sqrt 3 +10}-\sqrt[3]{6\sqrt 3 -10}

Evaluate the expression, without using a calculator.


The answer is 2.00.

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1 solution

Naren Bhandari
May 19, 2018

Let m = a 3 b 3 = 6 3 + 10 3 6 3 10 3 m 3 = ( a 3 b 3 ) 3 = a b 3 a b 3 ( a 3 b 3 ) m 3 + ( a b 3 ) m = a b m = \sqrt[3] {a} - \sqrt[3]{b} =\sqrt[3] {6\sqrt 3 +10} - \sqrt[3]{6\sqrt 3 -10} \\ m^3 =\,( \sqrt[3]a - \sqrt[3]b)^3 = a -b -3\sqrt[3]{ab}\,(\sqrt[3]a-\sqrt[3]b) \\ m^3 + (\sqrt[ 3]{ab})m = a-b Replugging the value we get m 3 + 3 ( 36.3 100 3 ) m = 20 m 3 + 6 m 20 = 0 ( m 2 ) ( m 2 + 2 m + 10 ) = 0 m^3 + 3(\sqrt[3]{36.3 -100}) m=20 \\ m^3 +6m -20 =0 \\ (m-2)(m^2+2m+10) =0 Equating we have m = 2 m =2 and Δ \Delta of quadratic equation is < 0 <0 so no real roots exists . Hence m = 2 m=\boxed{2}

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