f(x)f(x) is such that f′′(x)=18x−4f''(x) = 18x - 4. Given that f(0)=1f(0) = 1 and f(2)=9f(2) = 9,

  1. (a)

    find f(x)f(x).

  2. (b)

    Show that x+1x + 1 is a factor.

  3. (c)

    Solve f(x)=0f(x) = 0.

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Worked solution

5(a)

1. Integrate $f''(x)$ to find $f'(x)$

f′(x)=∫(18x−4) dx=9x2−4x+cf'(x) = \int (18x - 4) \, dx = 9x^2 - 4x + c

2. Integrate $f'(x)$ to find the general form of $f(x)$

f(x)=∫(9x2−4x+c) dx=3x3−2x2+cx+df(x) = \int (9x^2 - 4x + c) \, dx = 3x^3 - 2x^2 + cx + d

3. Use the condition $f(0) = 1$ to find $d$

f(0)=3(0)3−2(0)2+c(0)+d=1  ⟹  d=1f(0) = 3(0)^3 - 2(0)^2 + c(0) + d = 1 \implies d = 1
So, f(x)=3x3−2x2+cx+1f(x) = 3x^3 - 2x^2 + cx + 1

4. Use the condition $f(2) = 9$ to find $c$ and state the final function

f(2)=3(2)3−2(2)2+c(2)+1=9f(2) = 3(2)^3 - 2(2)^2 + c(2) + 1 = 9
24−8+2c+1=924 - 8 + 2c + 1 = 9
17+2c=917 + 2c = 9
2c=−8  ⟹  c=−42c = -8 \implies c = -4
Therefore, f(x)=3x3−2x2−4x+1f(x) = 3x^3 - 2x^2 - 4x + 1

Answer

f(x)=3x3−2x2−4x+1f(x) = 3x^3 - 2x^2 - 4x + 1

5(b)

1. State the function $f(x)$ found in part (a)

From part (a), f(x)=3x3−2x2−4x+1f(x) = 3x^3 - 2x^2 - 4x + 1.

2. Evaluate $f(-1)$ to test if $x+1$ is a factor

f(−1)=3(−1)3−2(−1)2−4(−1)+1f(-1) = 3(-1)^3 - 2(-1)^2 - 4(-1) + 1
f(−1)=−3−2+4+1f(-1) = -3 - 2 + 4 + 1
f(−1)=0f(-1) = 0

3. Conclude using the Factor Theorem

Since f(−1)=0f(-1) = 0, by the Factor Theorem, x+1x + 1 is a factor of f(x)f(x).

Answer

Since f(−1)=0f(-1) = 0, by the Factor Theorem, x+1x + 1 is a factor of f(x)f(x).

5(c)

1. State the polynomial $f(x)$ from part (a)

From part (a), f(x)=3x3−2x2−4x+1f(x) = 3x^3 - 2x^2 - 4x + 1.

2. Factorise $f(x)$ using the factor $x+1$ from part (b)

Since x+1x+1 is a factor, we can write f(x)=(x+1)(ax2+bx+c)f(x) = (x+1)(ax^2+bx+c).
By comparing coefficients or long division:
3x3−2x2−4x+1=(x+1)(3x2−5x+1)3x^3 - 2x^2 - 4x + 1 = (x+1)(3x^2 - 5x + 1)

3. Solve the quadratic factor equal to zero

Set 3x2−5x+1=03x^2 - 5x + 1 = 0.
Using the quadratic formula:
x=−(−5)±(−5)2−4(3)(1)2(3)x = \frac{-(-5) \pm \sqrt{(-5)^2 - 4(3)(1)}}{2(3)}
x=5±25−126x = \frac{5 \pm \sqrt{25 - 12}}{6}
x=5±136x = \frac{5 \pm \sqrt{13}}{6}

4. State all solutions to $f(x) = 0$

The solutions are x=−1x = -1 and x=5±136x = \frac{5 \pm \sqrt{13}}{6}.

Answer

x=−1,5±136x = -1, \frac{5 \pm \sqrt{13}}{6}

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