Activity 1
Factorise each of the following completely:

  1. 12x2 + 17x + 6
  2. 5x2 – 23x – 10
  3. 9x2 + 5x – 4
  4. 12x2 – 11x + 2
  5. 5x2 – 45
  6. 2x3 + 16
  7. 6x3 – 13x2 + 5x
    [16]

Solutions

  1. 12x2 + 17x + 6
    = 12x2 + 9x + 8x +6...........12 × 6 = 72 and 72 = 9 × 8 (9 + 8 = 17)
    = 3x(4x + 3) + 2(4x + 3)
    = (4x + 3)(3x + 2) (2)
  2. 5x2 – 23x – 10
    = 5x2 – 25x + 2x – 10............5 × –10 = –50 and 50 = 25 × 2 (–25 + 2 = –23)
    = 5x(x – 5) + 2(x – 5)
    = (x – 5)(5x + 2) (2)
  3. 9x2 + 5x – 4
    = 9x2 + 9x – 4x – 4..............9 × –4 = –36 and 36 = 9 × 4 (9 – 4 = 5)
    = 9x(x + 1) – 4(x + 1)
    = (x + 1)(9x – 4) (2)
  4. 12x2 – 11x + 2
    = 12x2 – 3x – 8x + 2..............12 × 2 = 24 and 24 = 8 × 3 (–8 – 3 = –11)
    = 3x(4x – 1) – 2(4x – 1)
    = (4x – 1)(3x – 2) (2)
  5. 5x2 – 45............................Common factor of 5
    = 5(x2 – 9) ............................Difference of two squares
    = 5(x – 3)(x + 3) (2)
  6. 2x3 + 16............................Common factor of 2
    = 2(x3 + 8) ............................Sum of two cubes
    = 2(x + 2)( x2 – 2x + 4)  (3)
  7. 6x3 – 13x2 + 5x............................Common factor of x
    = x(6x2 – 13x + 5) 3............................Trinomial factorising
    = x(6x2 – 3x – 10x + 5) .............. 6 × 5 = 30 and 30 = 10 × 3 (–3 – 10 = –13)
    = x[3x(2x – 1) – 5(2x –1)]
    = x[(2x – 1)(3x – 5)] (3)
    [16] 

Activity 2
Solve for x:

  1. x(x + 3) = 0
  2. x(2x – 5) = 12
  3. 2x2  + x − 6 = 0
  4. 2x2 = 32
  5. 3x + 1/x = 4, x ≠ 0
  6. 2√x − 3   = x − 3

[22]
If (A) × (B) = 0, then either A = 0 OR B = 0.

Solutions

  1. x(x + 3) = 0 We have a product = 0. Therefore put each factor = 0
    x = 0 or x + 3 = 0
    x = 0 3 or x = –3 (2)
  2. x(2x – 5) = 12 We need a product = 0. Therefore, multiply out brackets and write in standard form with all the terms on one side and equal to 0
    (2x + 3)(x – 4) = 0 Factorise
    2x + 3 = 0 or x – 4 = 0 Put each factor = 0
    2x = –3 or x = 4
    x = -3/2   x = 4 (2)
  3. 2x2  + x − 6 = 0
    (2x − 3)(x + 2) = 0 33
    ∴ 2x = 3 or x = − 2 Find the solutions by putting each factor equal to zero
    ∴ x = -3/2 or x =  −2 (4)
  4. 2x2 = 32
    2x2 – 32 = 0 Write in standard form with all the terms on one side and equal to 0
    x2 – 16 = 0  Divide every term on both sides by 2
    (x + 4)(x – 4) = 0 3 Factorise (the difference of two squares)
    ∴ x + 4 = 0 or x – 4 = 0
    ∴ x = –4  or ∴ x = 4  (4)
  5. 3x + 1/x = 4, x ≠ 0 Multiply through by x to get rid of the denominator
    3x2 + 1 = 4x Write in standard form with all the terms on one side and equal to 0
    3x2 – 4x + 1 = 0 Factorise (the trinomial)
    (3x – 1)(x – 1) = 0 
    ∴ 3x – 1 = 0 or x – 1 = 0
    ∴ 3x = 1 or x = 1
    ∴ x = 1/or ∴ x = 1 (5)
  6. 2 √x − 3   = x − 3 Square both sides
    (2 √x − 3)2  =  (x − 3)2
    4(x − 3 )  =  ( x − 3 ) ( x − 3 ) 
    4x − 12 =  x2  − 6x + 9
    0 =  x2  − 10x + 21 
    0 =  ( x − 7 ) ( x − 3 )
     ∴ x − 7 = 0 or x − 3 = 0
     ∴ x = 7  or x = 3  (5)
    [22] 

Check your answers:
x = 7
LHS = 2 √7 − 3   = 2 √4  = 2(2) = 4 RHS = 7 − 3 = 4  ∴ x = 7 is a solution
x = 3
LHS = 2 √3 − 3   = 2 √0  = 0 RHS = 3 − 3 = 0   ∴ x = 3 is a solution
If a quadratic equation cannot be factorised, there are other ways to find the roots or solutions. Sometimes the solutions do not exist!

Activity 3

  1. What term can be added to the following equations to make a complete square?
    1. 0 = x2 – 8x + ?
    2. y = x2 + 9x + ?
    3. y = x2 -b/a x + ?
  2. Solve for x by using the method of completing the square.
    1. –3x2 + 5x + 4 = 0
    2. ax2 + bx + c = 0 [17]

1

Activity 4: Interpret a graph
Solve for x (correct to two decimal places):
4x2 – 8x = 7
2x (3x + 5) – 11 = 0 [9]

Solutions
1. 4x2 – 8x = 7 Write the equation in standard form (ax2 + bx + c = 0)
4x2 – 8x = 7 = 0 List the values of a, b and c
a = 4; b = – 8; c = –7 Write down the formula
x =  − b ± √b2 − 4ac Substitute the values for a, b and c into the formula.
                  2
x = – ( –8 ) ± √(–8 ) 2 – 4(4)(–7) Simplify the value under the square root sign
                         2(4)  
x = 8 ± √176    Separate the positive and negative value of the square root
           8
x = 8 + √176  or x = 8 – √176    Answers in surd form
           8                        8  
x = 2,66  3  or x = –0,66 3 Answers correct to two decimal places (5)
2. 2x(3x + 5) – 11 = 0 Write the equation in standard form
6x2 + 10x – 11 = 0 (ax2 + bx + c = 0)
x = –10 ± √100 + 26
                12
= –10 ± √364
               12
= –5 ± √91
             6 (4)
[9] 
These roots are irrational. Unless the question asks for decimal values,
leave them in surd form (the square root form).

Activity 5
Solve for x if

  1. (x+3)(x – 5) ≤–12
  2. –x ≤ 2x2 – 3 [10]

Solutions

  1. (x+3)(x – 5) ≤ –12
    x 2 – 2x – 15 +12 ≤ 0 Get into the standard form (ax2 + bx + c ≤ 0)
    x 2 – 2x –3 ≤ 0 3 
    Factorise the trinomial:
    (x –3)(x + 1) ≤ 0 3 
    Critical values:
    x = 3 and x –1
    We now indicate 3 and –1 on a number line.
    9
    We know that the expression x2 – 2x –3 = 0 at x = 3 and x = –1. We can indicate this on the number line.
    10
    Our next step is to choose values less than –1, values between –1 and 3 and values greater than 3 and substitute it into the expression x 2 – 2x –3. If the answer is positive, then we indicate + on the number line. If the answer is negative, we indicate – on the number line
    (If x = –10, then 2(–10)² +(–10) – 3 = 187 > 0  ∴ +
    If x = 0, then 2(0)² – 0 –3 = –3 < 0 ∴ –
    If x = 3, then 2(3)² + 3 – 3 = 18 > 0 ∴ +)
    11
    We have to solve for x where x2 – 2x –3 ≤ 0. The solution on the number line is the interval where we see zero and a negative. This happen when the x values are less than or equal to 3 and are also more than or equal to –1.
    12
    Therefore the solution is : − 1 ≤ × ≤ 3 (5)
  2. –x ≤ 2x2 – 3
    –2x2 – x + 3 ≤ 0 Get into the standard form (ax2 + bx + c ≤ 0)
    –2x +≥ 
     -1     -1   -1    -1 
    Divide both sides by –1 to change the coefficient of x2 to a positive
    2x2 + x – 3  ≥  0 3 Factorise the trinomial
    (2x + 3)(x – 1)  ≥  0 3 
    Critical values:
    x = –3/2 and x = 1
    We now indicate –3/2 and 1 on a number line.
    13
    We know that the expression 2x2 + x – 3 = 0 at x = –3/and x = 1.
    We can indicate this on the number line.
    14
    Our next step is to choose values less than –3/2, values between–3/and 1 and values greater than 1 and substitute it into the expression 2x2 + x – 3. If the answer is positive, then we indicate + on the number line. If the answer is negative, we indicate – on the number line
    (Whenever we multiply or divide an inequality by a negative, the inequality sign changes i.e. the less or equal to sign changes to a greater or equal to sign.
    If x = –10, then (–10)² 2(–10) – 3 =117 > 0  ∴ +
    If x = 1, then (1)² – 2(1) – 3 =–4 < 0 ∴ –
    If x = 5, then (5)² –2(5) – 3 = 12 > 0 ∴  +)
    We have to solve for x where 2x2 + x – 3 ≥ 0. The solution on the number line is the interval where we see zero and a positive. This happens for the x values less than or equal to –3/and for the x values greater than or equal to 1.
    16
    Therefore the solution is : x ≤ − –3/or x ≥ 1  33  (5)

    OR ALTERNATIVE METHOD by using a rough sketch of the parabola:
    –x < 2x2 – 3 Get into the standard form ax2 + bx + c < 0
    –2x2– x + 3 < 0 Divide both sides by –1.
    –2x x + > 0 This is necessary to draw the rough sketch of a “positive” parabola
     -1    -1   -1
    2x2 + x – 3 > 0 3 With 0 on RHS
    (2x + 3)(x – 1) > 0  3  Factorise LHS
    Critical values of x: –3/and = 1 Make a rough sketch of a parabola
    17
    If (***)(***) > 0 (it means where y is positive)
    Read off the x values of the graph above the x-axis (5)
    [10] 

Activity 6
Solve the following equations simultaneously.
1. 2x + y = 3 and x2 + y + x = y2
2. y =  − 6   − 2 and y =  − 3x + 2
          x + 1   
[14]

Solutions

  1. 2x + y = 3 ..................... eqn (1)
    x2 + y + x = y2 .............. eqn (2)
    y = –2x + 3  .............. eqn (3) Use the linear equation (1) to write y alone on one side of the equation.
    Substitute eqn (3) into eqn (2), to eliminate the y variable.
    x2 + (–2x + 3) + x = (–2x + 3)2  3  Simplify both sides.
    x2 – x + 3 = 4x2 – 12x + 9 Factorise the trinomial.
    0 = 3x2 – 11x + 6  
    0 = (3x – 2)(x – 3) 
    ∴ 3x – 2 = 0 or x – 3 = 0
    ∴ x = 2/3 or ∴ x = 3 
    20
  2. y =   − 6   − 2 and y =  − 3x + 2
          x + 1
    y = − 6    − 2.....................(eqn 1)
         x + 1 
    y = –3x + 2.......................(eqn 2)
    y is alone on one side of both equations.
    ∴ y =  − 6   − 2 = –3x + 2 ............LCD = x + 1
            x + 1
    ∴ –6 –2(x + 1) = –3x(x + 1) + 2(x + 1)
    ∴ –6 –2x –2 = –3x2 – 3x + 2x + 2
    ∴ 3x2 – x – 10 = 0 
    ∴ (3x + 5)(x – 2) = 0 
    ∴ x = -5/3 or x = 2
    21

[14] 

Activity 7

  1. Show that the roots of x2 – 2x – 7 = 0, are irrational, without solving the equation. (3)
  2. Show that x2 + x + 1 = 0 has no real roots. (3)
  3. If x = 2 is a root of the equation 3x2 – 5x – 2k = 0, determine the value of k. (2)
  4. The solutions to a quadratic equation are: x = 5 ± √12 − 3a .
    For which value(s) of a will the equation have equal roots. (3)
  5. Determine the value(s) of k for which the equation 3x2 + (k + 2) x + k = 0 has equal roots (4)
    [15]

Solutions

  1. a = 1; b = –2 ; c = –7
    Δ = b2 – 4ac = (–2)2 – 4(1)(–7)  
    = 4 + 28
    = 32  
    ∴ The roots will be irrational  
    (Δ > 0 and not a perfect square) (3)
  2. a = 1; b = 1; c = 1
    Δ = b2 – 4ac = (1)2 – 4 (1)(1) 
    = 1 – 4 = – 3 
    ∴ There are no real roots
    (Δ < 0 )  (3)
  3. If 2 is a root of the equation, then x = 2. Therefore, we can substitute x = 2 into the equation.
    3x2 – 5x – 2k = 0
      ∴ 3(2)2 – 5(2) –2k = 0  
      ∴ 12 − 10 − 2k = 0
      ∴ 2k = 2
    ∴ k = 1  3 (2)
  4. The equation will have equal roots if Δ = 0
    Δ = 12 – 3a
    0  3 = 12 − 3a 
      − 12 = − 3a
    ∴ a = 4  (3)
  5. 3x2 + (k + 2)x + k = 0
    ∴ a = 3 ; b = (k + 2) ; c = k
    ∴ Δ = b2 – 4ac
    = (k + 2)2 – 4(3)(k) 
    = k2 + 4k + 4 – 12k
    = k2 – 8k + 4
    For equal roots the Δ = 0  
    ∴ k2 – 8k + 4 = 0
    ∴ k = 8± √(–8 ) 2 –4(1)(4)
                        2(1)
    ∴ k = 8± √48
                2
    ∴ k = 7,46 or k = –0,54  (4)
    [15] 

 

Last modified on Friday, 03 September 2021 07:56