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Exercise 948Algebra › Equations and radicalsL1Added 3 Nov 2026

A cube root equation

#cube root#cubing#real solutions

Statement

Solve in R\mathbb{R} the equation
x+13+x−13=2x3.\sqrt[3]{x+1}+\sqrt[3]{x-1}=\sqrt[3]{2x}.

Hint

Cube both sides using (a+b)3=a3+b3+3ab(a+b)(a+b)^3=a^3+b^3+3ab(a+b).

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Solution

Notation. Let a=x+13a=\sqrt[3]{x+1}, b=x−13b=\sqrt[3]{x-1}, c=2x3c=\sqrt[3]{2x}. The equation is a+b=ca+b=c.

Cubing. (a+b)3=a3+b3+3ab(a+b)=c3(a+b)^3=a^3+b^3+3ab(a+b)=c^3, i.e.
(x+1)+(x−1)+3(x+1)(x−1)3⋅c=2x,(x+1)+(x-1)+3\sqrt[3]{(x+1)(x-1)}\cdot c=2x,
i.e.
2x+3(x2−1)3⋅2x3=2x.2x+3\sqrt[3]{(x^2-1)}\cdot\sqrt[3]{2x}=2x.
So 32x(x2−1)3=03\sqrt[3]{2x(x^2-1)}=0, hence 2x(x2−1)=02x(x^2-1)=0, so x=0x=0 or x=±1x=\pm1.

Check.
- x=0x=0: 13+−13=1−1=0=03\sqrt[3]1+\sqrt[3]{-1}=1-1=0=\sqrt[3]0. OK.
- x=1x=1: 23+0=23\sqrt[3]2+0=\sqrt[3]2. OK.
- x=−1x=-1: 0+−23=−23=−230+\sqrt[3]{-2}=-\sqrt[3]2=\sqrt[3]{-2}. OK.

Conclusion. x∈{−1,0,1}x\in\{-1,0,1\}.