Show that the roots of the equation ax2+(3a+b)x+3b=0ax^2 + (3a + b)x + 3b = 0 are real for all real values of aa and bb. Find the range of values of mm for which the line y=mx−3y = mx - 3 will never cut the curve y2=4x−6y−34y^2 = 4x - 6y - 34.

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Worked solution

3(a)

1. Write down the expression for the discriminant of the quadratic equation.

For the equation ax2+(3a+b)x+3b=0ax^2 + (3a + b)x + 3b = 0, the discriminant is:
Δ=(3a+b)2−4(a)(3b)\Delta = (3a + b)^2 - 4(a)(3b)

2. Expand and simplify the discriminant.

Δ=9a2+6ab+b2−12ab\Delta = 9a^2 + 6ab + b^2 - 12ab
Δ=9a2−6ab+b2\Delta = 9a^2 - 6ab + b^2

3. Factorise into a perfect square and conclude.

Δ=(3a−b)2\Delta = (3a - b)^2
Since (3a−b)2≥0(3a - b)^2 \ge 0 for all real values of aa and bb, the discriminant is always non-negative. Therefore, the roots of the equation are real for all real values of aa and bb.

Answer

Shown that discriminant ≥0\ge 0.

3(b)

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

Substitute y=mx−3y = mx - 3 into y2=4x−6y−34y^2 = 4x - 6y - 34:
(mx−3)2=4x−6(mx−3)−34(mx - 3)^2 = 4x - 6(mx - 3) - 34

2. Expand and rearrange into a standard quadratic equation in $x$

m2x2−6mx+9=4x−6mx+18−34m^2x^2 - 6mx + 9 = 4x - 6mx + 18 - 34
m2x2−6mx+9=4x−6mx−16m^2x^2 - 6mx + 9 = 4x - 6mx - 16
m2x2−4x+25=0m^2x^2 - 4x + 25 = 0

3. Apply the discriminant condition for no intersection

For the line to never cut the curve, the quadratic equation must have no real roots, so b2−4ac<0b^2 - 4ac < 0.
Here, a=m2a = m^2, b=−4b = -4, c=25c = 25.
(−4)2−4(m2)(25)<0(-4)^2 - 4(m^2)(25) < 0
16−100m2<016 - 100m^2 < 0

4. Solve the quadratic inequality for $m$

100m2>16100m^2 > 16
m2>16100=425m^2 > \frac{16}{100} = \frac{4}{25}
m<−25 or m>25m < -\frac{2}{5} \text{ or } m > \frac{2}{5}

Answer

m<−14m < -\frac{1}{4}

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