Correct Answer :
81
Solution :
The correct answer is 81.
We are given the quadratic expression:
where .
For a quadratic expression to be positive for all real numbers , the following two conditions must be satisfied:
1. The coefficient of must be positive: . Since , this condition is always satisfied.
2. The discriminant () of the quadratic expression must be negative: .
Let's calculate the discriminant:
For :
or equivalently, .
The elements are chosen from the set . The total number of outcomes for is:
Now, let's analyze the cases based on the value of to find the number of favorable outcomes:
Case 1:
Here, .
We require .
The pairs that do not satisfy this condition (i.e., ) are:
- since
- since
- since
There are 3 such pairs. Since there are possible pairs of in total, the number of favorable pairs for is:
Case 2:
Here, .
We require .
Let's find the pairs that satisfy :
- If , then can be 3 or 4 (giving products 9 and 12) ⇒ 2 pairs:
- If , then can be 3 or 4 (giving products 12 and 16) ⇒ 2 pairs:
Thus, the number of favorable pairs for is 4.
Case 3:
Here, .
We require .
Since the maximum possible value of is , there are no pairs that satisfy this condition. Thus, the number of favorable pairs is 0.
Case 4:
Here, .
We require .
Since the maximum value of is 16, the number of favorable pairs is 0.
Now, let's sum up the favorable outcomes:
The probability is given by:
Since 17 is a prime number and 64 is not divisible by 17, and are coprime.
Thus, we compute the required value:
Access expert-curated educational resources and study materials—completely free.
Create, conduct, and manage professional online assessments with Mindyard. Perfect for teachers and institutes.
Copyright © 2026 Mindyard. All Rights Reserved.