Express y=4x−4x2−3y = 4x - 4x^2 - 3 in the form p(x+q)2+rp(x + q)^2 + r where pp, qq and rr are constants. Hence, explain whether 4x−4x2−3=04x - 4x^2 - 3 = 0 has any real solutions.

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Worked solution

6(a)

1. Rearrange the terms and factor out the coefficient of $x^2$ from the $x$ terms.

y=−4x2+4x−3y = -4x^2 + 4x - 3
y=−4(x2−x)−3y = -4(x^2 - x) - 3

2. Complete the square for the expression inside the bracket.

y=−4[(x−12)2−(−12)2]−3y = -4\left[ \left(x - \frac{1}{2}\right)^2 - \left(-\frac{1}{2}\right)^2 \right] - 3
y=−4[(x−12)2−14]−3y = -4\left[ \left(x - \frac{1}{2}\right)^2 - \frac{1}{4} \right] - 3

3. Expand and simplify to reach the final form.

y=−4(x−12)2+1−3y = -4\left(x - \frac{1}{2}\right)^2 + 1 - 3
y=−4(x−12)2−2y = -4\left(x - \frac{1}{2}\right)^2 - 2

Answer

y=−4(x−12)2−2y = -4\left(x - \frac{1}{2}\right)^2 - 2

6(b)

1. Express the quadratic equation using the completed square form from part (a)

From part (a), 4x−4x2−3=−4(x−12)2−24x - 4x^2 - 3 = -4\left(x - \frac{1}{2}\right)^2 - 2.
So the equation becomes −4(x−12)2−2=0-4\left(x - \frac{1}{2}\right)^2 - 2 = 0.

2. Explain why there are no real solutions based on the maximum value

For any real xx, (x−12)2⩾0\left(x - \frac{1}{2}\right)^2 \geqslant 0, which means −4(x−12)2⩽0-4\left(x - \frac{1}{2}\right)^2 \leqslant 0.
Therefore, −4(x−12)2−2⩽−2-4\left(x - \frac{1}{2}\right)^2 - 2 \leqslant -2.
Since the maximum value of the expression is −2-2, it can never be equal to 00. Hence, the equation has no real solutions.

Answer

No real solutions, since the maximum value of the expression is −2-2, which is less than 00.

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