Algebra · real student question

Solve the inequality x^2 * x - 5 > 0.

Question

Solve the inequality

x2x5>0x^2\cdot x-5>0

Step-by-step solution

  1. Simplify the expression first. Multiplying powers with the same base adds the exponents: x2x=x2+1=x3x^2\cdot x=x^{2+1}=x^3. So the inequality is cubic, not quadratic:

    x35>0x^3-5>0

    Misreading x2xx^2x as x2x^2 or as 2x22x^2 changes the problem completely.

  2. Isolate the cubic term. Add 55 to both sides:

    x3>5x^3>5

  3. Take the cube root of both sides. This is where cubes differ crucially from squares: the function tt3t\mapsto t^3 is strictly increasing on all of R\mathbb{R} and one-to-one, so cube-rooting preserves the inequality and needs no ±\pm and no case split:

    x>53x>\sqrt[3]{5}

    By contrast, x2>5x^2>5 would give the two-part answer x>5x>\sqrt5 or x<5x<-\sqrt5.

  4. Evaluate the bound.

    53=1.7099761.71\sqrt[3]{5}=1.709976\ldots\approx1.71

    A quick check on the size: 1.73=4.9131.7^3=4.913 (just under 55) and 1.713=5.0002111.71^3=5.000211 (just over), so the root sits between them ✓.

  5. Write the solution set. A single ray, open at the left because the inequality is strict:

    x>53,(53, )x>\sqrt[3]{5},\qquad\left(\sqrt[3]{5},\ \infty\right)

  6. Verify by scanning. Evaluating x35x^3-5 at 10,00110{,}001 points from 5-5 to 55 and comparing the sign against x>53x>\sqrt[3]{5} gives agreement at every point ✓ — including the negatives, where the cubic is strongly negative and correctly excluded.

Answer

x>531.71x>\sqrt[3]{5}\approx 1.71

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