- #1
jaus tail
- 615
- 48
Homework Statement
A quadratic eqn of form ax2 + bx + c = 0 is selected. The values of a, b and c are distinct and selected from 1, 2, 3, 4, 6, 8, 9. What is probability of chosen equation to have equal roots?
Homework Equations
root = (-b +/- Sqrt(b2-4ac)) / (2a)
For equal roots b2 = 4ac[/B]
The Attempt at a Solution
well i found out that for b2 = 4ac, the conditions are
b=6, a = 1, c = 9.
b=6, a = 9, c = 1.
Probability = no. of likely events/total no. of events.
Probability of A and B to occur = P(A)*P(B)
Case 1---
So probability of value of a to be 6 is 1/7. (as total 7 numbers are there)
Probability of value of b to be 1 is 1/7
Probability of c to be 9 is 1/7.
Case 2---
Likewise Probability of a to be 6 is again 1/7
P(b=9) = 1/7
P(c=1) = 1/7
P(equal roots) = P(case 1) + P(case 2)
= 1/73 + 1/73
=2/343
But in solutions it goes like: there are two ways in which condition can be achieved for equal roots.
Total ways = 7P3.
So P(equal roots) = 2/(7P3)
Why is my method wrong? And why should it be 7P3 and not 7C3?[/B]