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Quadratics — Solving simultaneous equations (one linear and one quadratic) 16 problems

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📘 Notes

Simultaneous Equations (One Linear and One Quadratic) — Year 12

We solve by substituting the linear equation into the quadratic.

1. Method Summary

Given one linear and one quadratic equation:

  1. Rearrange the linear equation to make \(x=\) or \(y=\).
  2. Substitute into the quadratic to form a single equation.
  3. Solve the quadratic (factor or quadratic formula).
  4. Substitute values back into the linear equation to find the other variable.
Always solve the quadratic completely (there are usually two solutions).

2. Example 1 — Intersection of a Line and a Curve

Solve:

\[ y = x + 1 \qquad\text{and}\qquad y = x^2 - 3 \]

Step 1: Substitute \(y = x + 1\) into \(y = x^2 - 3\):

\[ x + 1 = x^2 - 3 \Rightarrow x^2 - x - 4 = 0 \]

Step 2: Solve the quadratic:

\[ x = \frac{1 \pm \sqrt{1 + 16}}{2} = \frac{1 \pm \sqrt{17}}{2} \]

Step 3: Substitute into \(y = x + 1\):

\[ y = \frac{1 \pm \sqrt{17}}{2} + 1 = \frac{3 \pm \sqrt{17}}{2} \]

Solutions: \[ \left(\frac{1 + \sqrt{17}}{2},\ \frac{3 + \sqrt{17}}{2}\right) \quad\text{and}\quad \left(\frac{1 - \sqrt{17}}{2},\ \frac{3 - \sqrt{17}}{2}\right) \]

3. Example 2 — Two Points of Intersection

Solve:

\[ 2x + y = 4 \qquad\text{and}\qquad y = x^2 + x - 6 \]

Step 1: Rearrange the linear equation:

\[ y = 4 - 2x \]

Step 2: Substitute into the quadratic:

\[ 4 - 2x = x^2 + x - 6 \Rightarrow x^2 + 3x - 10 = 0 \]

Step 3: Solve:

\[ (x + 5)(x - 2) = 0 \Rightarrow x = -5,\ x = 2 \]

Step 4: Substitute into \(y = 4 - 2x\):

For \(x = 2\): \(y = 0\) For \(x = -5\): \(y = 14\)

Solutions: \((2, 0)\) and \((-5, 14)\)

4. Tips for Year 12

  • Always solve the quadratic fully — expect two solutions.
  • If answers look messy (with square roots), they are usually correct.
  • Factor if possible, otherwise use the quadratic formula.
  • Solutions show points where the line and the curve intersect.
  • If no real solutions, the line does not meet the curve.

5. Try These

Solve each pair:

  1. \(y = 3x + 2\), \(y = x^2 - 5x + 4\)
  2. \(x + y = 1\), \(y = x^2 - 4\)
  3. \(2y - x = 1\), \(y = x^2 + 3x\)

Give answers in coordinate form: \((x, y)\).

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