The equation of a curve is y=3x2+5x+1y = 3x^2 + 5x + 1. Find the set of values of xx for which the curve lies completely above the line y−3x=2y - 3x = 2. Find the range of values of mm for which the equation mx2+2m=3x(4−x)mx^2 + 2m = 3x(4 - x) has real roots.

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Worked solution

2(a)

1. Set up the inequality for the curve lying above the line

The curve lies above the line when ycurve>yliney_{\text{curve}} > y_{\text{line}}.
3x2+5x+1>3x+23x^2 + 5x + 1 > 3x + 2

2. Rearrange into a standard quadratic inequality and factorise

3x2+2x−1>03x^2 + 2x - 1 > 0
(3x−1)(x+1)>0(3x - 1)(x + 1) > 0

3. Solve the quadratic inequality

The critical values are x=13x = \frac{1}{3} and x=−1x = -1.
Since the quadratic is positive, the solution is:
x<−1 or x>13x < -1 \text{ or } x > \frac{1}{3}

Answer

x<−1x < -1 or x>13x > \frac{1}{3}

2(b)

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

mx2+2m=12x−3x2mx^2 + 2m = 12x - 3x^2
(m+3)x2−12x+2m=0(m + 3)x^2 - 12x + 2m = 0

2. Apply the discriminant condition for real roots.

For real roots, b2−4ac⩾0b^2 - 4ac \geqslant 0.
(−12)2−4(m+3)(2m)⩾0(-12)^2 - 4(m + 3)(2m) \geqslant 0

3. Expand and simplify the inequality.

144−8m(m+3)⩾0144 - 8m(m + 3) \geqslant 0
144−8m2−24m⩾0144 - 8m^2 - 24m \geqslant 0
8m2+24m−144⩽08m^2 + 24m - 144 \leqslant 0
m2+3m−18⩽0m^2 + 3m - 18 \leqslant 0

4. Solve the quadratic inequality for $m$.

(m+6)(m−3)⩽0(m + 6)(m - 3) \leqslant 0
−6⩽m⩽3-6 \leqslant m \leqslant 3

Answer

−6⩽m⩽3-6 \leqslant m \leqslant 3

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