Find the range of values of xx which satisfies x2+2x+2x2−5x+6≥0.\frac{x^2 + 2x + 2}{x^2 - 5x + 6} \ge 0. Show that the line y=x+ky = x + k always intersects the curve xy=y+2xy = y + 2 at two distinct points for all real values of kk.

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Worked solution

8(a)

1. Analyse the sign of the numerator

For the numerator x2+2x+2x^2 + 2x + 2:
Discriminant Δ=22−4(1)(2)=4−8=−4\Delta = 2^2 - 4(1)(2) = 4 - 8 = -4.
Since Δ<0\Delta < 0 and the coefficient of x2x^2 is 1>01 > 0, x2+2x+2>0x^2 + 2x + 2 > 0 for all real values of xx.

2. Set up the inequality for the denominator

Since the numerator is always strictly positive, for the fraction x2+2x+2x2−5x+6≥0\frac{x^2 + 2x + 2}{x^2 - 5x + 6} \ge 0 to hold, the denominator must be strictly positive (it cannot be zero).
x2−5x+6>0x^2 - 5x + 6 > 0

3. Solve the quadratic inequality for the denominator

(x−2)(x−3)>0(x - 2)(x - 3) > 0
The critical values are x=2x = 2 and x=3x = 3.

4. Determine the final range of values for x

Since the quadratic curve opens upwards, the expression is positive outside the roots.
x<2 or x>3x < 2 \text{ or } x > 3

Answer

x<2x < 2 or x>3x > 3

8(b)

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

Substitute y=x+ky = x + k into xy=y+2xy = y + 2:
x(x+k)=(x+k)+2x(x + k) = (x + k) + 2

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

x2+kx=x+k+2x^2 + kx = x + k + 2
x2+(k−1)x−(k+2)=0x^2 + (k - 1)x - (k + 2) = 0

3. Find the discriminant of the quadratic equation

Δ=b2−4ac\Delta = b^2 - 4ac
Δ=(k−1)2−4(1)[−(k+2)]\Delta = (k - 1)^2 - 4(1)[-(k + 2)]
Δ=k2−2k+1+4k+8\Delta = k^2 - 2k + 1 + 4k + 8
Δ=k2+2k+9\Delta = k^2 + 2k + 9

4. Show that the discriminant is always positive for all real $k$

Complete the square for the discriminant:
Δ=(k+1)2−1+9\Delta = (k + 1)^2 - 1 + 9
Δ=(k+1)2+8\Delta = (k + 1)^2 + 8
Since (k+1)2≥0(k + 1)^2 \ge 0 for all real kk, (k+1)2+8≥8>0(k + 1)^2 + 8 \ge 8 > 0.
Therefore, Δ>0\Delta > 0 for all real kk, which means the equation always has two distinct real roots. Thus, the line always intersects the curve at two distinct points.

Answer

Since k2+4>0k^2 + 4 > 0 for all real kk, the discriminant is always positive, hence the line always intersects the curve at two distinct points.

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