OCR FP1 2015 June — Question 10 10 marks

Exam BoardOCR
ModuleFP1 (Further Pure Mathematics 1)
Year2015
SessionJune
Marks10
PaperDownload PDF ↗
Mark schemeDownload PDF ↗
TopicRoots of polynomials
TypeSubstitution to find new equation
DifficultyStandard +0.8 This is a Further Maths FP1 question requiring substitution to form a new equation and then using symmetric functions of roots. Part (i) is straightforward substitution, but part (ii) requires expressing α⁴+β⁴+γ⁴ in terms of elementary symmetric functions (using Newton's identities or power sum formulas), which is non-trivial and goes beyond standard A-level techniques. The multi-step algebraic manipulation needed elevates this above average difficulty.
Spec4.05b Transform equations: substitution for new roots

10 The cubic equation \(x ^ { 3 } + 4 x + 3 = 0\) has roots \(\alpha , \beta\) and \(\gamma\).
  1. Use the substitution \(x = \sqrt { u }\) to obtain a cubic equation in \(u\).
  2. Find the value of \(\alpha ^ { 4 } + \beta ^ { 4 } + \gamma ^ { 4 } + \alpha \beta \gamma\).

Question 10(i):
AnswerMarks Guidance
AnswerMarks Guidance
B1Use given substitution correctly in LHS of equation
M1Rearrange and square to eliminate \(\sqrt{u}\) or multiply by \(u^{\frac{3}{2}} + 4u^{\frac{1}{2}} - 3\)
\(u^3 + 8u^2 + 16u - 9 = 0\)A1 Obtain correct answer, must be an equation \(= 0\)
[3]
Question 10(ii):
Either:
AnswerMarks Guidance
AnswerMarks Guidance
\(\alpha\beta\gamma = -3\)B1 State or use correct result
\(\sum\alpha^2 = -8\quad \sum\alpha^2\beta^2 = 16\)B1 B1 Use correct result, using correct (i) or using an identity involving \(\sum\alpha=0,\ \sum\alpha\beta=4\)
\(\left(\sum\alpha^2\right)^2 = \sum\alpha^4 + 2\sum\alpha^2\beta^2\)M1* Obtain an identity connecting \(\sum\alpha^4\) and \(\left(\sum\alpha^2\right)^2\)
A1Obtain a correct answer
\(29\)DM1 Use their values in their expression
A1Obtain correct answer, c.w.o.
[7]
Or:
AnswerMarks Guidance
AnswerMarks Guidance
\(\alpha\beta\gamma = -3\)B1 State or use correct result
\(\sum\alpha=0,\ \sum\alpha\beta=4,\ \sum\alpha^2=-8,\ \sum\alpha^2\beta^2=16\)B1 B1 Use any 2 correct B1, other 2 correct B1
\(\sum\alpha^4 + 4\sum\alpha^2\cdot\sum\alpha\beta + 6\sum\alpha^2\beta^2 + 8\alpha\beta\gamma\sum\alpha\)M1 Expand \((\alpha+\beta+\gamma)^4\) and get expression involving symmetric functions only
A1Obtain correct expression
M1Use their values in their expression
A1Obtain correct answer, c.w.o.
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## Question 10(i):

| Answer | Marks | Guidance |
|--------|-------|----------|
| | B1 | Use given substitution correctly in LHS of equation |
| | M1 | Rearrange and square to eliminate $\sqrt{u}$ or multiply by $u^{\frac{3}{2}} + 4u^{\frac{1}{2}} - 3$ |
| $u^3 + 8u^2 + 16u - 9 = 0$ | A1 | Obtain correct answer, must be an equation $= 0$ |
| **[3]** | | |

---

## Question 10(ii):

**Either:**

| Answer | Marks | Guidance |
|--------|-------|----------|
| $\alpha\beta\gamma = -3$ | B1 | State or use correct result |
| $\sum\alpha^2 = -8\quad \sum\alpha^2\beta^2 = 16$ | B1 B1 | Use correct result, using correct (i) or using an identity involving $\sum\alpha=0,\ \sum\alpha\beta=4$ |
| $\left(\sum\alpha^2\right)^2 = \sum\alpha^4 + 2\sum\alpha^2\beta^2$ | M1* | Obtain an identity connecting $\sum\alpha^4$ and $\left(\sum\alpha^2\right)^2$ |
| | A1 | Obtain a correct answer |
| $29$ | DM1 | Use their values in their expression |
| | A1 | Obtain correct answer, c.w.o. |
| **[7]** | | |

**Or:**

| Answer | Marks | Guidance |
|--------|-------|----------|
| $\alpha\beta\gamma = -3$ | B1 | State or use correct result |
| $\sum\alpha=0,\ \sum\alpha\beta=4,\ \sum\alpha^2=-8,\ \sum\alpha^2\beta^2=16$ | B1 B1 | Use any 2 correct B1, other 2 correct B1 |
| $\sum\alpha^4 + 4\sum\alpha^2\cdot\sum\alpha\beta + 6\sum\alpha^2\beta^2 + 8\alpha\beta\gamma\sum\alpha$ | M1 | Expand $(\alpha+\beta+\gamma)^4$ and get expression involving symmetric functions only |
| | A1 | Obtain correct expression |
| | M1 | Use their values in their expression |
| | A1 | Obtain correct answer, c.w.o. |

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10 The cubic equation $x ^ { 3 } + 4 x + 3 = 0$ has roots $\alpha , \beta$ and $\gamma$.\\
(i) Use the substitution $x = \sqrt { u }$ to obtain a cubic equation in $u$.\\
(ii) Find the value of $\alpha ^ { 4 } + \beta ^ { 4 } + \gamma ^ { 4 } + \alpha \beta \gamma$.

\hfill \mbox{\textit{OCR FP1 2015 Q10 [10]}}