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Given that the equation [tex]k x^2 + 6 k x + 5 = 0[/tex], where [tex]k[/tex] is a non-zero constant, has no real roots, find the range of possible values of [tex]k[/tex]. (4 marks)

Sagot :

To determine the range of possible values of [tex]\( k \)[/tex] such that the equation [tex]\( k x^2 + 6k x + 5 = 0 \)[/tex] has no real roots, let's follow these steps:

1. Understand the condition for no real roots:
For a quadratic equation [tex]\( ax^2 + bx + c = 0 \)[/tex] to have no real roots, its discriminant must be less than zero. The discriminant [tex]\( \Delta \)[/tex] is given by:
[tex]\[ \Delta = b^2 - 4ac \][/tex]

2. Identify coefficients in the given quadratic equation:
The given equation is [tex]\( k x^2 + 6k x + 5 = 0 \)[/tex]. Here, the coefficients are:
[tex]\[ a = k, \quad b = 6k, \quad c = 5 \][/tex]

3. Compute the discriminant:
Substitute [tex]\( a \)[/tex], [tex]\( b \)[/tex], and [tex]\( c \)[/tex] into the discriminant formula:
[tex]\[ \Delta = (6k)^2 - 4(k)(5) \][/tex]
[tex]\[ \Delta = 36k^2 - 20k \][/tex]

4. Set up the inequality for no real roots:
For the quadratic equation to have no real roots, the discriminant must be less than zero:
[tex]\[ 36k^2 - 20k < 0 \][/tex]

5. Solve the inequality:
Factorize the quadratic expression:
[tex]\[ 36k^2 - 20k = 4k(9k - 5) \][/tex]
Hence, the inequality becomes:
[tex]\[ 4k(9k - 5) < 0 \][/tex]
Since [tex]\( k \)[/tex] is a non-zero constant and must be positive:
[tex]\[ 4k(9k - 5) < 0 \][/tex]
Analyze the factor [tex]\( 4k \)[/tex]:
- [tex]\( 4k \)[/tex] is always positive if [tex]\( k > 0 \)[/tex].
Analyze the factor [tex]\( (9k - 5) \)[/tex]:
- [tex]\( (9k - 5) < 0 \)[/tex] when [tex]\( 9k < 5 \)[/tex].

Therefore, the inequality [tex]\( 4k(9k - 5) < 0 \)[/tex] holds true when:
[tex]\[ 9k - 5 < 0 \implies k < \frac{5}{9} \][/tex]

6. Conclusion:
Combine the conditions found above. Since [tex]\( k \)[/tex] is a non-zero, positive constant:
[tex]\[ 0 < k < \frac{5}{9} \][/tex]

Thus, the range of possible values of [tex]\( k \)[/tex] such that the equation [tex]\( k x^2 + 6k x + 5 = 0 \)[/tex] has no real roots is:
[tex]\[ 0 < k < \frac{5}{9} \][/tex]
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