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Algebra Level 4

If x + y + z = 2196 x+y+z=2196

x 3 + y + z = 2076 \sqrt [ 3 ]{ x } +y+z=2076

x + y 3 + z = 1860 x+\sqrt [ 3 ]{ y } +z=1860

x + y + z 3 = 480 x+y+\sqrt [ 3 ]{ z } =480

and if x x , y y and z z are positive integers,

Find x 2 3 + y 2 3 + z 2 3 { x }^{ \frac { 2 }{ 3 } }+{ y }^{ \frac { 2 }{ 3 } }+{ z }^{ \frac { 2 }{ 3 } }


The answer is 218.

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1 solution

Ong Zi Qian
Feb 26, 2018

Since x 3 + y + z \sqrt [ 3 ]{ x } +y+z , x + y 3 + z x+\sqrt [ 3 ]{ y } +z and x + y + z 3 x+y+\sqrt [ 3 ]{ z } give us integer answer.

\Rightarrow Let x 3 = a \sqrt [3]{x}=a , y 3 = b \sqrt [3]{y}=b and z 3 = c \sqrt [3]{z}=c , we can rewrite the question to

a 3 + b 3 + c 3 = 2196 a^3+b^3+c^3 = 2196 --------(1)

a + b 3 + c 3 = 2076 a+b^3+c^3 = 2076 -------(2)

a 3 + b + c 3 = 1860 a^3+b+c^3 = 1860 -------(3)

a 3 + b 3 + c = 480 a^3+b^3+c =480 -------(4)

(1) - (2) , we get a 3 a = 120 a^3-a=120

Factorizes both size of the equation, a ( a 1 ) ( a + 1 ) = 2 3 × 3 × 5 a ( a + 1 ) ( a 1 ) = 4 × 5 × 6 a = 5 a(a-1)(a+1) = 2^3 \times 3 \times 5\\a(a+1)(a-1)=4 \times 5 \times 6 \\\boxed{a=5}

We can find the value of b b and c c by the same method.

(1) - (3) , we get b 3 b = 336 b^3-b=336

Factorizes both size of the equation, b ( b 1 ) ( b + 1 ) = 2 4 × 3 × 7 b ( b + 1 ) ( b 1 ) = 6 × 7 × 8 b = 7 b(b-1)(b+1) = 2^4 \times 3 \times 7\\b(b+1)(b-1)=6 \times 7 \times 8 \\\boxed{b=7}

(1) - (4) , we get c 3 c = 1716 c^3-c=1716

Factorizes both size of the equation, c ( c 1 ) ( c + 1 ) = 2 2 × 3 × 11 × 13 c ( c + 1 ) ( c 1 ) = 11 × 12 × 13 c = 12 c(c-1)(c+1) = 2^2 \times 3 \times 11 \times 13\\c(c+1)(c-1)=11 \times 12 \times 13 \\\boxed{c=12}

Therefore, x 2 3 + y 2 3 + z 2 3 = a 2 + b 2 + c 2 = 5 2 + 7 2 + 1 2 2 = 25 + 49 + 144 = 218 {x}^{\frac {2}{3} }+{ y }^{ \frac {2}{3}}+{ z }^{ \frac{2}{3}}\\=a^2+b^2+c^2\\=5^2+7^2+12^2\\=25+49+144\\=\boxed{218}

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