Let f(x) = x³ + 4x² + x - 6 - Edexcel - A-Level Maths Pure - Question 7 - 2007 - Paper 2
Question 7
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)−6=−8+16−2−6=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:
Divide the leading term: xx3=x2.
Multiply (x + 2) by x² giving x3+2x2.
Subtract from f(x):
f(x)−(x3+2x2)=2x2+x−6.
Repeat the process: Divide 2x2 by x yielding 2x.
Multiply (x + 2) by 2x to get 2x2+4x and subtract:
2x2+x−6−(2x2+4x)=−3x−6.
Finally, divide −3x by x to get −3 and complete the division:
−3(x+2)=−3x−6.
Thus, we have:
f(x)=(x+2)(x2+2x−3).
Now, we factor x2+2x−3:
x2+2x−3=(x+3)(x−1).
Therefore, the complete factorization is:
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)(x−1)=0.
The solutions are found by setting each factor equal to zero:
x+2=0
=> x=−2
x+3=0
=> x=−3
x−1=0
=> x=1.
Thus, the solutions to the equation are:
x=−2,−3,1.