Find the exact roots of the following quadratic equations.

  1. (a)

    x2+3x+2=0x^2 + 3x + 2 = 0

  2. (b)

    −x2−2x+3=0-x^2 - 2x + 3 = 0

  3. (c)

    −x22+2x−1=0-\frac{x^2}{2} + 2x - 1 = 0

  4. (d)

    23x2+x−14=0\frac{2}{3}x^2 + x - \frac{1}{4} = 0

  5. (e)

    −πx2+x+π=1-\pi x^2 + x + \pi = 1

  6. (f)

    x22+6x+3=0\frac{x^2}{2} + \sqrt{6}x + 3 = 0

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Worked solution

2(a)

1. Factorise the quadratic expression to its linear factors.

x2+3x+2=0x^2 + 3x + 2 = 0
(x+1)(x+2)=0(x + 1)(x + 2) = 0

2. Equate each factor to zero to solve for the exact roots.

x+1=0  ⟹  x=−1x + 1 = 0 \implies x = -1
x+2=0  ⟹  x=−2x + 2 = 0 \implies x = -2

Answer

x=−1x = -1 or x=−2x = -2

2(b)

1. Multiply the equation by $-1$ to make the leading coefficient positive for easier factorisation.

x2+2x−3=0x^2 + 2x - 3 = 0

2. Factorise the quadratic expression.

(x+3)(x−1)=0(x + 3)(x - 1) = 0

3. Determine the roots by setting each factor to zero.

x+3=0  ⟹  x=−3x + 3 = 0 \implies x = -3
x−1=0  ⟹  x=1x - 1 = 0 \implies x = 1

Answer

x=−3 or x=1x = -3 \text{ or } x = 1

2(c)

1. Multiply the entire equation by $-2$ to remove the fraction and make the $x^2$ coefficient positive.

−x22+2x−1=0-\frac{x^2}{2} + 2x - 1 = 0
x2−4x+2=0x^2 - 4x + 2 = 0

2. Apply the quadratic formula to find the roots.

x=−(−4)±(−4)2−4(1)(2)2(1)x = \frac{-(-4) \pm \sqrt{(-4)^2 - 4(1)(2)}}{2(1)}
x=4±16−82x = \frac{4 \pm \sqrt{16 - 8}}{2}

3. Simplify the surd to obtain the exact roots in their simplest form.

x=4±82x = \frac{4 \pm \sqrt{8}}{2}
x=4±222x = \frac{4 \pm 2\sqrt{2}}{2}
x=2±2x = 2 \pm \sqrt{2}

Answer

x=2−2x = 2 - \sqrt{2} or x=2+2x = 2 + \sqrt{2}

2(d)

1. Eliminate the fractions by multiplying the equation by the lowest common multiple of the denominators.

12(23x2+x−14)=012 \left(\frac{2}{3}x^2 + x - \frac{1}{4}\right) = 0
8x2+12x−3=08x^2 + 12x - 3 = 0

2. Apply the quadratic formula to find the roots.

x=−(12)±(12)2−4(8)(−3)2(8)x = \frac{-(12) \pm \sqrt{(12)^2 - 4(8)(-3)}}{2(8)}
x=−12±144+9616x = \frac{-12 \pm \sqrt{144 + 96}}{16}
x=−12±24016x = \frac{-12 \pm \sqrt{240}}{16}

3. Simplify the surd to obtain the exact final roots.

240=16×15=415\sqrt{240} = \sqrt{16 \times 15} = 4\sqrt{15}
x=−12±41516x = \frac{-12 \pm 4\sqrt{15}}{16}
x=−3±154x = \frac{-3 \pm \sqrt{15}}{4}

Answer

x=−3+154 or x=−3−154x = \frac{-3 + \sqrt{15}}{4} \text{ or } x = \frac{-3 - \sqrt{15}}{4}

2(e)

1. Rearrange the given equation into the standard quadratic form $ax^2 + bx + c = 0$.

−πx2+x+π=1-\pi x^2 + x + \pi = 1

πx2−x+1−π=0\pi x^2 - x + 1 - \pi = 0

2. Apply the quadratic formula using $a = \pi$, $b = -1$, and $c = 1 - \pi$.

x=−(−1)±(−1)2−4(π)(1−π)2(π)x = \frac{-(-1) \pm \sqrt{(-1)^2 - 4(\pi)(1 - \pi)}}{2(\pi)}

x=1±1−4π+4π22πx = \frac{1 \pm \sqrt{1 - 4\pi + 4\pi^2}}{2\pi}

3. Recognise the discriminant as a perfect square to simplify the square root.

Observe that 4π2−4π+1=(2π−1)24\pi^2 - 4\pi + 1 = (2\pi - 1)^2.

x=1±(2π−1)22πx = \frac{1 \pm \sqrt{(2\pi - 1)^2}}{2\pi}

x=1±(2π−1)2πx = \frac{1 \pm (2\pi - 1)}{2\pi}

4. Evaluate the two cases to find the exact roots.

Case 1:
x=1+(2π−1)2π=2π2π=1x = \frac{1 + (2\pi - 1)}{2\pi} = \frac{2\pi}{2\pi} = 1

Case 2:
x=1−(2π−1)2π=2−2π2π=2(1−π)2π=1−ππx = \frac{1 - (2\pi - 1)}{2\pi} = \frac{2 - 2\pi}{2\pi} = \frac{2(1 - \pi)}{2\pi} = \frac{1 - \pi}{\pi}

Answer

x=1 or x=1−ππx = 1 \text{ or } x = \frac{1 - \pi}{\pi}

2(f)

1. Substitute the coefficients into the quadratic formula

a=12a = \frac{1}{2}, b=6b = \sqrt{6}, c=3c = 3

x=−6±(6)2−4(12)(3)2(12)x = \frac{-\sqrt{6} \pm \sqrt{(\sqrt{6})^2 - 4\left(\frac{1}{2}\right)(3)}}{2\left(\frac{1}{2}\right)}

2. Simplify the discriminant to find the exact root

x=−6±6−61x = \frac{-\sqrt{6} \pm \sqrt{6 - 6}}{1}
x=−6x = -\sqrt{6}

Answer

x=−6x = -\sqrt{6}

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