Practicing Success

Target Exam

CUET

Subject

-- Mathematics - Section B1

Chapter

Probability

Question:

Each coefficient in the equation $ax^2 + bx + c = 0 $ is determined by throwing an ordinary six faced die. The probability that the equation will have real roots, is

Options:

$\frac{42}{216}$

$\frac{41}{216}$

$\frac{43}{216}$

$\frac{39}{216}$

Correct Answer:

$\frac{43}{216}$

Explanation:

Since each of the coefficient a, b and c can take the values from 1 to 6.

∴ Total numbers of equations = 6 × 6 × 6 = 216.

The roots of the equation $ax^2 + bx + c = 0 $ will be real if $b^2 - 4 ac ≥ 0 ⇒ b^2 ≥ 4 ac.$

The  favourable number of elementary events can be enumerated as follows:

 ac   

 a

  c   

 4ac   

 b(so that $ b^2 ≥ 4ac $

 No. of ways

 1

 1

 1

 4

 2, 3, 4, 5, 6

 1 × 5 = 5

 2

 $\left\{\begin{matrix}1\\2\end{matrix}\right.$

 2

 1

 8

 3, 4, 5, 6

 2 × 4 = 8

 3

 $\left\{\begin{matrix}1\\3\end{matrix}\right.$

3

 12

 4, 5, 6

 2 × 3 = 6

 4

 $\left\{\begin{matrix}1\\4\\2\end{matrix}\right.$

 4

1

2

 16

  4, 5, 6

 3×3=9

 5

  $\left\{\begin{matrix}1\\5\end{matrix}\right.$

 5

1

 20

 5, 6

 2 × 2 = 4

 6

  $\left\{\begin{matrix}1\\6\\2\\3\end{matrix}\right.$

 6

1

3

2

 24

 5, 6

 4 ×2 = 8

 7

 ac is not possible

 

 

 

 0

 8

  $\left\{\begin{matrix}2\\4\end{matrix}\right.$

 4

2

 32

 6

 2 ×1= 2

 9

 3

 3

 36

 6

 1

                                                         Total = 43

Since $b^2≥ 4ac$ and since the maximum value of $b^2$ is 36, therefore ac = 10, 11, 12 ...etc. is not possible.

∴ Total number of favourable elementary events = 43

Hence, required probability = $\frac{43}{216}$