A curve has the equation y=2x2−6x+cy = 2x^2 - 6x + c, where cc is a constant.
Find the value of cc for which the line y+2x=8y + 2x = 8 is a tangent to the curve.
Represent the solution set of 3(x2−5)>x−13(x^2 - 5) > x - 1 on the number line. Find the greatest value of integer pp for which −2x2+x−p-2x^2 + x - p has real roots for all real values of xx.

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Worked solution

5(a)

1. Form a quadratic equation by equating the line and the curve

From the line equation, y=8−2xy = 8 - 2x.
Substitute this into the curve equation:
8−2x=2x2−6x+c8 - 2x = 2x^2 - 6x + c
2x2−4x+c−8=02x^2 - 4x + c - 8 = 0

2. Apply the discriminant condition for a tangent

For the line to be a tangent to the curve, the quadratic equation must have equal roots, so the discriminant b2−4ac=0b^2 - 4ac = 0.
(−4)2−4(2)(c−8)=0(-4)^2 - 4(2)(c - 8) = 0

3. Solve for $c$

16−8(c−8)=016 - 8(c - 8) = 0
16−8c+64=016 - 8c + 64 = 0
80−8c=080 - 8c = 0
8c=808c = 80
c=10c = 10

Answer

c=10c = 10

5(b)

1. Expand and rearrange the inequality into standard quadratic form

3(x2−5)>x−13(x^2 - 5) > x - 1
3x2−15>x−13x^2 - 15 > x - 1
3x2−x−14>03x^2 - x - 14 > 0

2. Factorise the quadratic expression to find critical values

(3x−7)(x+2)>0(3x - 7)(x + 2) > 0
Critical values are x=73x = \frac{7}{3} and x=−2x = -2.

3. Determine the solution set and represent it on a number line

Since the inequality is >0> 0, the solution is x<−2x < -2 or x>73x > \frac{7}{3}.

(Draw a number line with open circles at −2-2 and 73\frac{7}{3}, with an arrow pointing left from −2-2 and an arrow pointing right from 73\frac{7}{3}.)

Answer

Number line showing x<−2x < -2 or x>73x > \frac{7}{3} (open circles at −2-2 and 73\frac{7}{3}, arrows pointing outwards).

5(c)

1. Identify coefficients and write the discriminant condition for real roots

For −2x2+x−p=0-2x^2 + x - p = 0: a=−2a = -2, b=1b = 1, c=−pc = -p.

For real roots, b2−4ac≥0b^2 - 4ac \geq 0.

2. Substitute and simplify the discriminant inequality

12−4(−2)(−p)≥01^2 - 4(-2)(-p) \geq 0
1−8p≥01 - 8p \geq 0
p≤18p \leq \frac{1}{8}

3. Select the greatest integer value of p

Since p≤18=0.125p \leq \dfrac{1}{8} = 0.125, the greatest integer value is
p=0.p = 0.

Answer

p=0p = 0

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