Find the range of values of the constant mm for which the curve y=(m−3)x2+mx+my = (m - 3)x^2 + mx + m lies completely above the xx-axis.

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Worked solution

2

1. State the conditions for the quadratic curve to lie completely above the x-axis

For the curve y=(m−3)x2+mx+my = (m - 3)x^2 + mx + m to lie completely above the xx-axis, it must be a U-shaped parabola that does not intersect the xx-axis.
Conditions:

  1. Coefficient of x2>0  ⟹  m−3>0x^2 > 0 \implies m - 3 > 0
  2. Discriminant <0  ⟹  b2−4ac<0< 0 \implies b^2 - 4ac < 0

2. Solve the condition for the coefficient of x^2

m−3>0  ⟹  m>3m - 3 > 0 \implies m > 3

3. Set up the discriminant inequality

b2−4ac<0b^2 - 4ac < 0
(m)2−4(m−3)(m)<0(m)^2 - 4(m - 3)(m) < 0

4. Solve the discriminant inequality

m2−4m2+12m<0m^2 - 4m^2 + 12m < 0
−3m2+12m<0-3m^2 + 12m < 0
−3m(m−4)<0-3m(m - 4) < 0
m(m−4)>0m(m - 4) > 0
m<0m < 0 or m>4m > 4

5. Combine the conditions to find the final range of m

Intersection of m>3m > 3 and (m<0m < 0 or m>4m > 4):
m>4m > 4

Answer

m>4m > 4

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