Advanced Equation #1

Algebra Level 3

What is the sum of all real roots of the equation x + 45 3 + 16 x 3 = 1 ? \sqrt[3]{x+45}+\sqrt[3]{16-x}=1? If you think there is no real root, submit 99999 as your answer.


The answer is -29.

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1 solution

Boi (보이)
Jun 14, 2017

Let a = x + 45 3 a=\sqrt[3]{x+45} and b = 16 x 3 b=\sqrt[3]{16-x} .

Then you'll see that

a 3 + b 3 = 61 and a + b = 1 a^3+b^3=61\quad \text{and} \quad a+b=1

Since ( a + b ) 3 = a 3 + b 3 + 3 a b ( a + b ) (a+b)^3=a^3+b^3+3ab(a+b) ,

a + b = 1 and a b = 20 a+b=1 \quad \text{and} \quad ab=-20

Therefore a = 5 a=5 or a = 4 a=-4 .

Substituting to the definition of a a , we get:

x = 80 x=80 or x = 109 x=-109 .

Since 125 3 + 64 3 = 1 \sqrt[3]{125}+\sqrt[3]{-64}=1 ,

The sum of all real roots of the given equation is 29 \boxed{-29} .

Very simple and elegant!

Zach Abueg - 3 years, 12 months ago

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Thank you! 👍

Boi (보이) - 3 years, 12 months ago

Yeah did the same.

Your problems are awesome, please keep posting more !

Harsh Shrivastava - 3 years, 12 months ago

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Awh thank you! You motivate me! XD

Boi (보이) - 3 years, 12 months ago

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