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Let f(x) = x³ + 4x² + x - 6 - Edexcel - A-Level Maths Pure - Question 7 - 2007 - Paper 2

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Let f(x) = x³ + 4x² + x - 6. (a) Use the factor theorem to show that (x + 2) is a factor of f(x). (b) Factorise f(x) completely. (c) Write down all the solutions ... show full transcript

Worked Solution & Example Answer:Let f(x) = x³ + 4x² + x - 6 - Edexcel - A-Level Maths Pure - Question 7 - 2007 - Paper 2

Step 1

Use the factor theorem to show that (x + 2) is a factor of f(x).

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Answer

To apply the factor theorem, we evaluate f(-2):

f(2)=(2)3+4(2)2+(2)6f(-2) = (-2)³ + 4(-2)² + (-2) - 6 =8+1626 = -8 + 16 - 2 - 6 =0. = 0.
Since f(-2) = 0, according to the factor theorem, (x + 2) is a factor of f(x).

Step 2

Factorise f(x) completely.

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Answer

Given that (x + 2) is a factor, we can divide f(x) by (x + 2). Performing polynomial long division:

  1. Divide the leading term: x3x=x2\frac{x^3}{x} = x^2.
  2. Multiply (x + 2) by x² giving x3+2x2x³ + 2x^2.
  3. Subtract from f(x): f(x)(x3+2x2)=2x2+x6f(x) - (x³ + 2x²) = 2x² + x - 6.
  4. Repeat the process: Divide 2x22x² by xx yielding 2x2x.
  5. Multiply (x + 2) by 2x2x to get 2x2+4x2x² + 4x and subtract: 2x2+x6(2x2+4x)=3x6.2x² + x - 6 - (2x² + 4x) = -3x - 6.
  6. Finally, divide 3x-3x by xx to get 3-3 and complete the division: 3(x+2)=3x6.-3(x + 2) = -3x - 6.
    Thus, we have: f(x)=(x+2)(x2+2x3).f(x) = (x + 2)(x^2 + 2x - 3).
    Now, we factor x2+2x3x^2 + 2x - 3: x2+2x3=(x+3)(x1).x^2 + 2x - 3 = (x + 3)(x - 1).
    Therefore, the complete factorization is: f(x)=(x+2)(x+3)(x1).f(x) = (x + 2)(x + 3)(x - 1).

Step 3

Write down all the solutions to the equation x³ + 4x² + x - 6 = 0.

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Answer

To find the solutions, we set f(x) = 0: (x+2)(x+3)(x1)=0(x + 2)(x + 3)(x - 1) = 0. The solutions are found by setting each factor equal to zero:

  1. x+2=0x + 2 = 0
    => x=2x = -2
  2. x+3=0x + 3 = 0
    => x=3x = -3
  3. x1=0x - 1 = 0
    => x=1x = 1. Thus, the solutions to the equation are: x=2,3,1.x = -2, -3, 1.

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