Find the range of values of kk for which the line y=kx+2y = kx + 2 intersects the curve xy−y=2xxy - y = 2x at two distinct points. Hence state the possible value(s) of kk for which the line y=kx+2y = kx + 2 is a tangent to the curve xy−y=2xxy - y = 2x.

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Worked solution

3((i))

1. Substitute the equation of the line into the equation of the curve

Substitute y=kx+2y = kx + 2 into xy−y=2xxy - y = 2x:
x(kx+2)−(kx+2)=2xx(kx + 2) - (kx + 2) = 2x

2. Simplify the equation into a standard quadratic form $ax^2 + bx + c = 0$

kx2+2x−kx−2=2xkx^2 + 2x - kx - 2 = 2x
kx2−kx−2=0kx^2 - kx - 2 = 0

3. Apply the discriminant condition for two distinct real roots

For the line to intersect the curve at two distinct points, the quadratic equation must have two distinct real roots, so b2−4ac>0b^2 - 4ac > 0.
(−k)2−4(k)(−2)>0(-k)^2 - 4(k)(-2) > 0
k2+8k>0k^2 + 8k > 0

4. Solve the quadratic inequality for $k$

k(k+8)>0k(k + 8) > 0
The critical values are k=0k = 0 and k=−8k = -8.
Since the quadratic curve y=k2+8ky = k^2 + 8k opens upwards, the inequality is satisfied for:
k<−8 or k>0k < -8 \text{ or } k > 0

Answer

k<−8 or k>0k < -8 \text{ or } k > 0

3((ii))

1. Identify the condition for the line to be a tangent using the discriminant from part (i)

From part (i), the intersection of y=kx+2y = kx + 2 and xy−y=2xxy - y = 2x gives the quadratic equation kx2−kx−2=0kx^2 - kx - 2 = 0.
For the line to be a tangent, the discriminant must be zero:
(−k)2−4(k)(−2)=0(-k)^2 - 4(k)(-2) = 0
k2+8k=0k^2 + 8k = 0

2. Solve for $k$ and reject the invalid value

k(k+8)=0k(k + 8) = 0
k=0 or k=−8k = 0 \text{ or } k = -8
If k=0k = 0, the equation kx2−kx−2=0kx^2 - kx - 2 = 0 becomes −2=0-2 = 0, which has no solution (the line y=2y=2 is an asymptote, not a tangent).
Therefore, k=−8k = -8.

Answer

k=−8k = -8

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