Algebra · real student question

Solve the equation sqrt(3x - 8) - sqrt(2x - 3) + sqrt(x - 1) = 0.

Question

Solve the equation

3x82x3+x1=0\sqrt{3x-8} - \sqrt{2x-3} + \sqrt{x-1} = 0

Step-by-step solution

  1. Find the domain before anything else. All three radicands must be non-negative: 3x83x \ge 8, 2x32x \ge 3, x1x \ge 1. The binding condition is x832.6667x \ge \tfrac{8}{3} \approx 2.6667, so any answer below that is automatically invalid.

  2. Regroup so both sides are non-negative. As written, the left side mixes signs and squaring it would be a mess. Move the negative term to the right:

    3x8+x1=2x3\sqrt{3x-8} + \sqrt{x-1} = \sqrt{2x-3}

    Now both sides are sums of principal roots, hence 0\ge 0, and squaring cannot introduce a sign error.

  3. Square once. Using (u+v)2=u2+2uv+v2(u+v)^2 = u^2 + 2uv + v^2:

    (3x8)+2(3x8)(x1)+(x1)=2x3(3x-8) + 2\sqrt{(3x-8)(x-1)} + (x-1) = 2x-3

    4x9+2(3x8)(x1)=2x3    (3x8)(x1)=3x4x - 9 + 2\sqrt{(3x-8)(x-1)} = 2x - 3 \;\Longrightarrow\; \sqrt{(3x-8)(x-1)} = 3 - x

  4. Record the new sign condition, then square again. The left side is 0\ge 0, so we need 3x03 - x \ge 0, i.e. x3x \le 3. Combined with the domain, any solution lies in [83,3]\left[\tfrac{8}{3},\,3\right] — a very narrow window. Squaring:

    3x211x+8=x26x+9    2x25x1=03x^2 - 11x + 8 = x^2 - 6x + 9 \;\Longrightarrow\; 2x^2 - 5x - 1 = 0

  5. Solve and filter. By the quadratic formula:

    x=5±25+84=5±334x = \frac{5 \pm \sqrt{25+8}}{4} = \frac{5 \pm \sqrt{33}}{4}

    That is x2.6861x \approx 2.6861 or x0.1861x \approx -0.1861. The negative value is far outside the domain (its radicands are negative), so only x=5+334x = \tfrac{5+\sqrt{33}}{4} can work — and note how tightly it sits inside [2.6667,3][2.6667,\,3].

  6. Verify in the original equation. At x=2.6861407x = 2.6861407: 3x8=0.058422=0.241706\sqrt{3x-8} = \sqrt{0.058422} = 0.241706, 2x3=2.372281=1.540221\sqrt{2x-3} = \sqrt{2.372281} = 1.540221, x1=1.686141=1.298515\sqrt{x-1} = \sqrt{1.686141} = 1.298515. Then 0.2417061.540221+1.298515=0.0000000.241706 - 1.540221 + 1.298515 = 0.000000 ✓. Symbolically the residual simplifies to exactly 00, so x=5+334x = \tfrac{5+\sqrt{33}}{4} is the unique solution.

Answer

x=5+3342.6861x = \frac{5+\sqrt{33}}{4} \approx 2.6861

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