Given that 4x2−xh−3=04x^2 - \frac{x}{h} - 3 = 0, where h≠0h \ne 0, show that there are no real values of hh for which the equation has equal real roots. Find the range of values of the positive constant kk for which the line y=kxy = kx lies completely above the curve y=−16x2+(2k−1)x−1y = -16x^2 + (2k - 1)x - 1.

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Worked solution

3(a)

1. Set up the discriminant condition for equal real roots.

For equal real roots, the discriminant b2−4ac=0b^2 - 4ac = 0.
Here, a=4a = 4, b=−1hb = -\frac{1}{h}, and c=−3c = -3.

2. Substitute the coefficients into the discriminant equation.

(−1h)2−4(4)(−3)=0\left(-\frac{1}{h}\right)^2 - 4(4)(-3) = 0
1h2+48=0\frac{1}{h^2} + 48 = 0

3. Show that the resulting equation has no real solutions for $h$.

1h2=−48\frac{1}{h^2} = -48
For any real value of h≠0h \ne 0, h2>0h^2 > 0, which means 1h2>0\frac{1}{h^2} > 0.
Since 1h2\frac{1}{h^2} cannot be negative, there are no real values of hh that satisfy this equation. Thus, the equation cannot have equal real roots.

Answer

Shown that 1h2=−48\frac{1}{h^2} = -48 has no real solutions.

3(b)

1. Set up the inequality for the line to lie completely above the curve

For the line y=kxy = kx to lie completely above the curve y=−16x2+(2k−1)x−1y = -16x^2 + (2k - 1)x - 1, we must have:
kx>−16x2+(2k−1)x−1kx > -16x^2 + (2k - 1)x - 1

2. Rearrange the inequality into standard quadratic form

16x2+kx−(2k−1)x+1>016x^2 + kx - (2k - 1)x + 1 > 0
16x2+(1−k)x+1>016x^2 + (1 - k)x + 1 > 0

3. Apply the discriminant condition for the quadratic to be always positive

For the quadratic expression to be always positive (since the coefficient of x2x^2 is 16>016 > 0), the discriminant must be less than zero:
b2−4ac<0b^2 - 4ac < 0
(1−k)2−4(16)(1)<0(1 - k)^2 - 4(16)(1) < 0
(1−k)2−64<0(1 - k)^2 - 64 < 0

4. Solve the quadratic inequality for $k$ and apply the given condition

k2−2k+1−64<0k^2 - 2k + 1 - 64 < 0
k2−2k−63<0k^2 - 2k - 63 < 0
(k−9)(k+7)<0(k - 9)(k + 7) < 0
−7<k<9-7 < k < 9
Since kk is a positive constant (k>0k > 0), the range of values is:
0<k<90 < k < 9

Answer

0<k<90 < k < 9

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