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Algebra · real student question

Solve -4.7714x^2 - 0.0884x + 10.014 = 0 for x.

Question

Solve

y=−4.7714x2−0.0884x+10.014y=-4.7714x^2-0.0884x+10.014

for the values of xx where y=0y=0.

Step-by-step solution

  1. Identify the coefficients. Setting y=0y=0 gives a quadratic in standard form with

    a=−4.7714,b=−0.0884,c=10.014a=-4.7714,\qquad b=-0.0884,\qquad c=10.014

    Because a<0a<0 the parabola opens downward, and since c>0c>0 the graph is already above the axis at x=0x=0 — so two real roots straddling the origin are expected.

  2. Compute the discriminant.

    Δ=b2−4ac=(−0.0884)2−4(−4.7714)(10.014)=0.007815+191.123=191.13101\Delta=b^2-4ac=(-0.0884)^2-4(-4.7714)(10.014)=0.007815+191.123=191.13101

    It is positive, confirming two distinct real roots. Note how small b2b^2 is compared with −4ac-4ac; the linear term barely influences the result.

  3. Apply the quadratic formula.

    x=−b±Δ2a=0.0884±191.13101−9.5428=0.0884±13.82501−9.5428x=\frac{-b\pm\sqrt{\Delta}}{2a}=\frac{0.0884\pm\sqrt{191.13101}}{-9.5428}=\frac{0.0884\pm 13.82501}{-9.5428}

    The denominator is negative, so the plus branch produces the smaller root — a sign trap specific to downward parabolas.

  4. Evaluate both branches.

    x1=0.0884+13.82501−9.5428=−1.45800,x2=0.0884−13.82501−9.5428=1.43947x_1=\frac{0.0884+13.82501}{-9.5428}=-1.45800,\qquad x_2=\frac{0.0884-13.82501}{-9.5428}=1.43947

  5. Check with Vieta’s formulas. The sum of the roots must be −ba=−−0.0884−4.7714=−0.018527-\tfrac{b}{a}=-\tfrac{-0.0884}{-4.7714}=-0.018527, and indeed −1.45800+1.43947=−0.01853  ✓-1.45800+1.43947=-0.01853\;\checkmark. The product must be ca=10.014−4.7714=−2.09876\tfrac{c}{a}=\tfrac{10.014}{-4.7714}=-2.09876, and (−1.45800)(1.43947)=−2.09875  ✓(-1.45800)(1.43947)=-2.09875\;\checkmark. Reporting the roots as −1.4576-1.4576 and 1.43801.4380 would fail both checks in the fourth decimal.

Answer

x≈−1.4580andx≈1.4395x\approx -1.4580\quad\text{and}\quad x\approx 1.4395

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