An Exercise on Quadratic Equations | Properties of Roots (Sum & Product, Discriminant)

Synopsis

This article covers the properties of roots of quadratic equations – an advanced topic in DSE Paper 1 Section A(2) and Section B worth 4–7 marks. You will learn the sum and product of roots, how to construct quadratic equations from given roots, how to solve parameter problems using root properties, and how to analyze the signs of roots using the discriminant and the sum/product. The article includes step-by-step worked examples, DSE exam techniques, practice questions, and a full set of exercise with solutions. These are high-level skills that distinguish top-performing students in the DSE exam.

Article

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 Quadratic Equations | Properties of Roots (Sum & Product, Discriminant)

Exercise

Section A(2) & Section B Practice

Question 1 MC
If \( \alpha \) and \( \beta \) are roots of \( x^2 + 8x - 3 = 0 \), find \( \alpha + \beta \).
A. \( -8 \)     B. \( -3 \)     C. \( 3 \)     D. \( 8 \)

Question 2 MC
If \( \alpha \) and \( \beta \) are roots of \( 4x^2 - 3x + 2 = 0 \), find \( \alpha\beta \).
A. \( \frac{1}{2} \)     B. \( -\frac{3}{4} \)     C. \( \frac{3}{4} \)     D. \( -\frac{1}{2} \)

Question 3 Short Answer
Construct a quadratic equation with roots \( 7 \) and \( -4 \).

Question 4 Short Answer
If \( \alpha \) and \( \beta \) are roots of \( x^2 - 5x + 3 = 0 \), find \( \frac{1}{\alpha} + \frac{1}{\beta} \).

Question 5 MC
The roots of \( x^2 + kx + 10 = 0 \) are \( 2 \) and \( 5 \). Find \( k \).
A. \( -7 \)     B. \( -3 \)     C. \( 3 \)     D. \( 7 \)

Question 6 Short Answer
If \( \alpha \) and \( \beta \) are roots of \( 2x^2 - 7x + 5 = 0 \), find \( \alpha^2 + \beta^2 \).

Question 7 MC
Which statement about the roots of \( x^2 + x - 6 = 0 \) is true?
A. Both roots are positive     B. Both roots are negative     C. One positive, one negative     D. No real roots

Question 8 Short Answer
If one root of \( x^2 - 10x + k = 0 \) is 3, find the other root and the value of \( k \).

Question 9 Short Answer
Determine the signs of the roots of \( x^2 - 2x - 8 = 0 \).

Question 10 Short Answer
If \( \alpha \) and \( \beta \) are roots of \( x^2 - 4x + 1 = 0 \), find \( \alpha^3 + \beta^3 \).

Answer Key for Exercise

Q1 A (\( -8 \))
Q2 A (\( \frac{1}{2} \))
Q3 \( x^2 - 3x - 28 = 0 \)
Q4 \( \frac{5}{3} \)
Q5 A (\( -7 \))
Q6 \( \frac{49}{4} - 5 = \frac{29}{4} \)
Q7 C (One positive, one negative)
Q8 Other root = 7, \( k = 21 \)
Q9 One positive, one negative (\( \Delta > 0 \), \( P = -8 < 0 \))
Q10 \( 52 \)
Common DSE Trap

When using \( \alpha^2 + \beta^2 = (\alpha + \beta)^2 - 2\alpha\beta \), remember to square \( \alpha + \beta \) first, then subtract \( 2\alpha\beta \). Don't mix up the order!