Everything Maths Grade 11

(Marvins-Underground-K-12) #1

CHAPTER 7. SOLVINGQUADRATIC EQUATIONS 7.5


Δ


Δ < 0 : imaginary roots Δ≥ 0 : real roots

Δ = 0


equal roots

Δ > 0


unequal roots

Δ a per-
fect square :
rational roots

Δ not a perfect
square : irra-
tional roots

Equal Roots (Δ = 0)
If Δ = 0, then the roots are equal and, from the formula,these are given by

x =−
b
2 a

(7.20)


Unequal Roots (Δ > 0 )
There will be two unequal roots if Δ > 0. The roots of f(x) are rational if Δ is a perfect
square (a number whichis the square of a rational number), since, in thiscase,


Δ is rational.
Otherwise, if Δ is not a perfect square,then the roots are irrational.

Imaginary Roots (Δ < 0 )

If Δ < 0 , then the solution to f(x) = ax^2 + bx + c = 0 contains the square root of a negative
number and therefore there are no real solutions. We therefore say thatthe roots of f(x) are
imaginary (the graph of the function f(x) does not intersect the x-axis).

See video: VMfba at http://www.everythingmaths.co.za

Extension: Theory of Quadratics - advanced exercises


Exercise 7 - 5



  1. [IEB, Nov. 2001, HG] Given: x^2 + bx− 2 + k(x^2 + 3x + 2) = 0, (k�=−1)
    (a) Show that the discriminant is given by:


Δ = k^2 + 6bk + b^2 + 8
(b) If b = 0, discuss the nature of the roots of the equation.
(c) If b = 2, find the value(s) of k for which the roots areequal.


  1. [IEB, Nov. 2002, HG] Show that k^2 x^2 +2 = kx− x^2 has non-real roots for allreal values
    for k.

  2. [IEB, Nov. 2003, HG] The equation x^2 + 12x = 3kx^2 + 2 has real roots.
    (a) Find the largest integral value of k.
    (b) Find one rational value of k, for which the above equation has rational roots.

  3. [IEB, Nov. 2003, HG] In the quadratic equation px^2 + qx+ r = 0, p, q and r are positive
    real numbers and form ageometric sequence. Discuss the nature of the roots.

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