Question Details

If probability that ax2 + 22bx + c > 0 xR where a, b, c { 1, 2, 3, 4 } is m n where m & n are coprime then ( m+n ) is

Options

A

81

B

18

C

78

D

17

Show Answer

Correct Answer :

Option A

81

Solution :

The correct answer is 81.

We are given the quadratic expression:
a x 2 + 2 2 b x + c > 0 x R
where a,b,c{1,2,3,4}.

For a quadratic expression ax2+Bx+C>0 to be positive for all real numbers x, the following two conditions must be satisfied:
1. The coefficient of x2 must be positive: a>0. Since a{1,2,3,4}, this condition is always satisfied.
2. The discriminant (D) of the quadratic expression must be negative: D<0.

Let's calculate the discriminant:
D = ( 2 2 b ) 2 - 4 a c
D = 8 b 2 - 4 a c

For D<0:
8 b 2 - 4 a c < 0
2 b 2 < a c
or equivalently, ac>2b2.

The elements a,b,c are chosen from the set {1,2,3,4}. The total number of outcomes for (a,b,c) is:
Total outcomes = 4 × 4 × 4 = 64

Now, let's analyze the cases based on the value of b to find the number of favorable outcomes:

Case 1: b=1
Here, 2b2=2(1)2=2.
We require ac>2.
The pairs (a,c) that do not satisfy this condition (i.e., ac2) are:
- (1,1) since 1×1=1
- (1,2) since 1×2=2
- (2,1) since 2×1=2
There are 3 such pairs. Since there are 4×4=16 possible pairs of (a,c) in total, the number of favorable pairs for b=1 is:
16 - 3 = 13

Case 2: b=2
Here, 2b2=2(2)2=8.
We require ac>8.
Let's find the pairs (a,c) that satisfy ac>8:
- If a=3, then c can be 3 or 4 (giving products 9 and 12) ⇒ 2 pairs: (3,3),(3,4)
- If a=4, then c can be 3 or 4 (giving products 12 and 16) ⇒ 2 pairs: (4,3),(4,4)
Thus, the number of favorable pairs for b=2 is 4.

Case 3: b=3
Here, 2b2=2(3)2=18.
We require ac>18.
Since the maximum possible value of ac is 4×4=16, there are no pairs that satisfy this condition. Thus, the number of favorable pairs is 0.

Case 4: b=4
Here, 2b2=2(4)2=32.
We require ac>32.
Since the maximum value of ac is 16, the number of favorable pairs is 0.

Now, let's sum up the favorable outcomes:
Total Favorable Outcomes = 13 + 4 + 0 + 0 = 17

The probability is given by:
P = m n = 17 64

Since 17 is a prime number and 64 is not divisible by 17, m=17 and n=64 are coprime.

Thus, we compute the required value:
m + n = 17 + 64 = 81

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