Roots of Quadratic Equations

Roots of Quadratic Equations

Roots of a quadratic equation are the values of variables/unknowns that satisfy the equation.

Example:
Determine whether 1, 2, and 3 are the roots of the quadratic equation x 2 5 x + 6 = 0 .

Answer:
When x = 1,
x 2 5 x + 6 = 0 ( 1 ) 2 5 ( 1 ) + 6 = 0 2 = 0
x = 1 does not satisfy the equation

When x = 2,
x 2 5 x + 6 = 0 ( 2 ) 2 5 ( 2 ) + 6 = 0 0 = 0
x = 2 satisfies the equation.

When x = 3
x 2 5 x + 6 = 0 ( 3 ) 2 5 ( 3 ) + 6 = 0 0 = 0
x = 3 satisfies the equation.

 Conclusion:
  1. 2 and 3 satisfy the equation x 2 5 x + 6 = 0 , hence there are the roots of the equation.
  2. 1 does not satisfy the equation x 2 5 x + 6 = 0 , hence it is NOT the root of the equation.

SPM Practice (Paper 1)

Question 11:
The quadratic equation x 2 4x1=2p(x5) , where p is a constant, has two equal roots. Calculate the possible values of p.

Solution:




Question 12:
Find the range of values of k for which the equation x 2 2kx+ k 2 +5k6=0 has no real roots.

Solution:




Question 13:
Find the range of values of p for which the equation 5 x 2 +7x3p=6 has no real roots.

Solution:

SPM Practice (Paper 1)

Question 6:
Write and simplify the equation whose roots are the reciprocals of the roots of 3 x 2 +2x1=0 , without solving the given equation.

Solution:





Question 7:
Find the value of p if one root of x 2 +px+8=0 is the square of the other.

Solution:





Question 8:
If one root of 2 x 2 +px+9=0 is twice the other, find the values of p.

Solution:


SPM Practice (Paper 1)


Question 1:
Solve the following quadratic equations by factorisation.
(a)  x 2 5x10=4 (b) 3x2 x 2 =0 (c) 11a=2 a 2 +12 (d)  2x+7 3x2 =x

Solution:








Question 2:
Solve the following quadratic equations by completing the square.
(a) 5 x 2 +10x3=0 (b) 2 x 2 5x6=0

Solution:





Finding the Sum of Roots (SoR) and Product of Roots (PoR)

2.6 Finding the Sum of Roots (SoR) and Product of Roots (PoR)



Example
Find the sums and products of the roots of the following equations.
a. x 2 + 7 x 3 = 0
b. x ( x 1 ) = 5 ( 1 x )

Answer:
(a)
x 2 + 7 x 3 = 0 a = 1 ,   b = 7 ,   c = 3 Sum of Roots α + β = b a = 7 1 = 7 Product of Roots α β = c a = 3 1 = 3

(b)
x ( x 1 ) = 5 ( 1 x ) x 2 x = 5 5 x x 2 x + 5 x 5 = 0 x 2 + 4 x 5 = 0 a = 1 ,   b = 4 ,   c = 5 Sum of Roots α + β = b a = 4 1 = 4 Product of Roots α β = c a = 5 1 5

2.2.2c Solving Quadratic Equations – Quadratic Formula

The quadratic equation a x 2 + b x + c = 0 can be solved by using the quadratic formula

b ± b 2 4 a c 2 a

Example 
Use the quadratic formula to find the solutions of the following equations.
a. x 2 + 5 x 24 = 0
b. x ( x + 4 ) = 10

Answer
(a)
For the equation x 2 + 5 x 24 = 0
a = 1, b = 5, c = -24

x = b ± b 2 4 a c 2 a x = ( 5 ) ± ( 5 ) 2 4 ( 1 ) ( 24 ) 2 ( 1 ) x = 5 ± 121 2 x = 8  or  x = 3

(b)
x ( x +  4 )   =  10 x 2 + 4 x 10 = 0 a = 1 ,    b = 4 ,    c = 10 x = b ± b 2 4 a c 2 a x = ( 4 ) ± ( 4 ) 2 4 ( 1 ) ( 10 ) 2 ( 1 ) x = 4 ± 56 2 x = 1.742  or  x = 5.742