Algebra · real student question

For a > 0, solve the inequality ax^2 - (2a - 1)x + 2 >= 2x.

Question

Let a>0a>0. Solve

ax2(2a1)x+22xax^{2}-(2a-1)x+2\ge 2x

Step-by-step solution

  1. Collect everything on the left. Subtracting 2x2x merges the two linear terms:

    ax2[(2a1)+2]x+20  ax2(2a+1)x+20ax^{2}-\left[(2a-1)+2\right]x+2\ge 0\ \Longrightarrow\ ax^{2}-(2a+1)x+2\ge 0

  2. Factor the quadratic. Try (x2)(ax1)(x-2)(ax-1) and expand to confirm:

    (x2)(ax1)=ax2x2ax+2=ax2(2a+1)x+2 (x-2)(ax-1)=ax^{2}-x-2ax+2=ax^{2}-(2a+1)x+2\ \checkmark

    so the inequality is (x2)(ax1)0(x-2)(ax-1)\ge 0.

  3. Normalise the leading coefficient. Because a>0a>0, dividing by aa does not flip the inequality, and ax1=a(x1a)ax-1=a\left(x-\tfrac1a\right):

    (x2)(x1a)0(x-2)\left(x-\frac{1}{a}\right)\ge 0

    The roots are 22 and 1a\tfrac1a; the parabola opens upward, so the solution is outside the roots — but which root is on the left depends on aa.

  4. Case 0<a<120<a<\tfrac12 (so 1a>2\tfrac1a>2). The roots in order are 2<1a2<\tfrac1a, giving

    (,2][1a,+)\left(-\infty,2\right]\cup\left[\frac{1}{a},+\infty\right)

    Check with a=14a=\tfrac14 (roots 22 and 44): at x=0x=0 the expression is 202\ge 0 ✓, at x=3x=3 it is 14<0-\tfrac14<0 ✗, at x=5x=5 it is 340\tfrac34\ge 0 ✓.

  5. Case a=12a=\tfrac12 (so 1a=2\tfrac1a=2). The roots coincide and the inequality becomes 12(x2)20\tfrac12(x-2)^{2}\ge 0, true for every real number:

    xRx\in\mathbb{R}

  6. Case a>12a>\tfrac12 (so 1a<2\tfrac1a<2). Now 1a\tfrac1a is the smaller root:

    (,1a][2,+)\left(-\infty,\frac{1}{a}\right]\cup\left[2,+\infty\right)

  7. Collect the three branches. The whole answer hinges on the single comparison 1a\tfrac1a versus 22, i.e. on whether a<12a<\tfrac12, a=12a=\tfrac12 or a>12a>\tfrac12 — the standard move for any parametric quadratic whose roots depend on the parameter.

Answer

{(,2][1a,+),0<a<12R,a=12(,1a][2,+),a>12\begin{cases}(-\infty,2]\cup\left[\dfrac{1}{a},+\infty\right), & 0<a<\dfrac12\\[6pt] \mathbb{R}, & a=\dfrac12\\[6pt] \left(-\infty,\dfrac{1}{a}\right]\cup[2,+\infty), & a>\dfrac12\end{cases}

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