Explain by completing the square that x2−2x+3x^2 - 2x + 3 is always positive for all real values of xx. Hence, find the range of values of pp if the inequality 3x2+px+3x2−2x+3>0\frac{3x^2 + px + 3}{x^2 - 2x + 3} > 0 is satisfied for all real values of xx.

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Worked solution

5(a)

1. Complete the square for the quadratic expression

x2−2x+3=(x−1)2−(−1)2+3x^2 - 2x + 3 = (x - 1)^2 - (-1)^2 + 3
=(x−1)2−1+3= (x - 1)^2 - 1 + 3
=(x−1)2+2= (x - 1)^2 + 2

2. Explain why the expression is always positive

For all real values of xx, (x−1)2≥0(x - 1)^2 \ge 0.
Therefore, (x−1)2+2≥2(x - 1)^2 + 2 \ge 2.
Since 2>02 > 0, the expression x2−2x+3x^2 - 2x + 3 is always positive.

Answer

Since (x−1)2≥0(x - 1)^2 \ge 0 for all real xx, (x−1)2+2≥2>0(x - 1)^2 + 2 \ge 2 > 0. Therefore, x2−2x+3x^2 - 2x + 3 is always positive.

5(b)

1. Deduce the condition for the numerator based on the denominator's sign.

From part (a), we know that x2−2x+3>0x^2 - 2x + 3 > 0 for all real values of xx.
For the fraction 3x2+px+3x2−2x+3\frac{3x^2 + px + 3}{x^2 - 2x + 3} to be strictly positive, the numerator must also be strictly positive for all real xx:
3x2+px+3>03x^2 + px + 3 > 0

2. Apply the discriminant condition for a quadratic expression to be always positive.

For the quadratic expression 3x2+px+33x^2 + px + 3 to be always positive, since the coefficient of x2x^2 is 3>03 > 0, its discriminant must be strictly less than zero:
b2−4ac<0b^2 - 4ac < 0
p2−4(3)(3)<0p^2 - 4(3)(3) < 0

3. Solve the quadratic inequality for $p$.

p2−36<0p^2 - 36 < 0
(p+6)(p−6)<0(p + 6)(p - 6) < 0
−6<p<6-6 < p < 6

Answer

−6<p<6-6 < p < 6

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