The equation of a polynomial is given by p(x)=4x3+x−5\mathrm{p}(x) = 4x^3 + x - 5.

  1. (i)

    Find the remainder when p(x)\mathrm{p}(x) is divided by (2x−1)(2x - 1).

  2. (ii)

    Show that the equation p(x)=0\mathrm{p}(x) = 0 has only one real root.

  3. (iii)

    Hence, solve the equation 23y+2+2y−5=02^{3y+2} + 2^y - 5 = 0.

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Worked solution

9(i)

1. Apply the Remainder Theorem by substituting $x = \frac{1}{2}$ into $\mathrm{p}(x)$

Remainder =p(12)= \mathrm{p}\left(\frac{1}{2}\right)
=4(12)3+(12)−5= 4\left(\frac{1}{2}\right)^3 + \left(\frac{1}{2}\right) - 5

2. Evaluate the expression to find the remainder

=4(18)+12−5= 4\left(\frac{1}{8}\right) + \frac{1}{2} - 5
=12+12−5= \frac{1}{2} + \frac{1}{2} - 5
=1−5= 1 - 5
=−4= -4

Answer

−4-4

9(ii)

1. Find a linear factor of $\mathrm{p}(x)$ using the Factor Theorem

Let p(x)=4x3+x−5\mathrm{p}(x) = 4x^3 + x - 5.
By trial and error,
p(1)=4(1)3+1−5=0\mathrm{p}(1) = 4(1)^3 + 1 - 5 = 0.
Therefore, (x−1)(x - 1) is a factor of p(x)\mathrm{p}(x).

2. Factorise the polynomial completely into a linear and a quadratic factor

4x3+x−5=(x−1)(Ax2+Bx+C)4x^3 + x - 5 = (x - 1)(Ax^2 + Bx + C)
By comparing coefficients:
A=4A = 4
C=5C = 5
Comparing coefficients of x2x^2:
B−A=0  ⟹  B=4B - A = 0 \implies B = 4
So, p(x)=(x−1)(4x2+4x+5)\mathrm{p}(x) = (x - 1)(4x^2 + 4x + 5).

3. Evaluate the discriminant of the quadratic factor

For the quadratic equation 4x2+4x+5=04x^2 + 4x + 5 = 0,
Discriminant, Δ=b2−4ac\Delta = b^2 - 4ac
Δ=42−4(4)(5)\Delta = 4^2 - 4(4)(5)
Δ=16−80=−64\Delta = 16 - 80 = -64

4. Conclude the number of real roots

Since Δ<0\Delta < 0, the quadratic equation 4x2+4x+5=04x^2 + 4x + 5 = 0 has no real roots.
Thus, x=1x = 1 is the only real root of p(x)=0\mathrm{p}(x) = 0.

Answer

Shown that x=1x=1 is the only real root.

9(iii)

1. Rewrite the exponential equation to reveal the polynomial structure

23y+2+2y−5=02^{3y+2} + 2^y - 5 = 0
22(23y)+2y−5=02^2(2^{3y}) + 2^y - 5 = 0
4(2y)3+2y−5=04(2^y)^3 + 2^y - 5 = 0

2. Use the substitution $x = 2^y$ and the result from part (ii)

Let x=2yx = 2^y. The equation becomes:
4x3+x−5=04x^3 + x - 5 = 0
This is p(x)=0\mathrm{p}(x) = 0. From part (ii), the only real root is x=1x = 1.

3. Solve for $y$

2y=12^y = 1
y=0y = 0

Answer

y=0y = 0

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