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Probability question

2024 · 5 Apr · Shift 2 · Q44
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  5. /2024 · 5 Apr · Shift 2 · Q44

Probability question

2024 · 5 Apr · Shift 2 · Q44

JEE MainMathematicsProbabilityMCQ+4 / −1
The coefficients a,b,c\mathrm{a}, \mathrm{b}, \mathrm{c}a,b,c in the quadratic equation ax2+bx+c=0\mathrm{a} x^2+\mathrm{bx}+\mathrm{c}=0ax2+bx+c=0 are from the set {1,2,3,4,5,6}\{1,2,3,4,5,6\}{1,2,3,4,5,6}. If the probability of this equation having one real root bigger than the other is p, then 216p equals :
  1. A
    38
  2. B
    76
  3. C
    57
  4. D
    19
View written solutionFree

Correct answer: A

  1. For the quadratic equation ax2+bx+c=0,ax^2+bx+c=0,ax2+bx+c=0, with a,b,c∈{1,2,3,4,5,6}a,b,c\in\{1,2,3,4,5,6\}a,b,c∈{1,2,3,4,5,6}, the total number of possible equations is 6×6×6=216.6\times 6\times 6=216.6×6×6=216.

  2. The equation has one real root bigger than the other means it has two distinct real roots. For a quadratic, this happens when the discriminant is positive: D=b2−4ac>0.D=b^2-4ac>0.D=b2−4ac>0.

  3. So we must count the number of ordered triples (a,b,c)(a,b,c)(a,b,c) such that b2>4ac.b^2>4ac.b2>4ac.

  4. Since b∈{1,2,3,4,5,6}b\in\{1,2,3,4,5,6\}b∈{1,2,3,4,5,6}, we check each value of bbb.


Case 1: b=1b=1b=1

b2=1b^2=1b2=1 Need: 4ac<14ac<14ac<1 Impossible since a,c≥1⇒4ac≥4a,c\ge 1\Rightarrow 4ac\ge 4a,c≥1⇒4ac≥4.

Number of favorable triples: 000.


Case 2: b=2b=2b=2

b2=4b^2=4b2=4 Need: 4ac<4⇒ac<14ac<4 \Rightarrow ac<14ac<4⇒ac<1 Impossible.

Number of favorable triples: 000.


Case 3: b=3b=3b=3

b2=9b^2=9b2=9 Need: 4ac<9⇒ac<94=2.254ac<9 \Rightarrow ac<\frac{9}{4}=2.254ac<9⇒ac<49​=2.25 So possible integer products are: ac=1,2.ac=1,2.ac=1,2. From a,c∈{1,2,3,4,5,6}a,c\in\{1,2,3,4,5,6\}a,c∈{1,2,3,4,5,6}, ordered pairs are:

  • (1,1)(1,1)(1,1)
  • (1,2)(1,2)(1,2)
  • (2,1)(2,1)(2,1)

Count = 333.


Case 4: b=4b=4b=4

b2=16b^2=16b2=16 Need: 4ac<16⇒ac<44ac<16 \Rightarrow ac<44ac<16⇒ac<4 So possible integer products are: ac=1,2,3.ac=1,2,3.ac=1,2,3. Ordered pairs:

  • ac=1ac=1ac=1: (1,1)(1,1)(1,1)
  • ac=2ac=2ac=2: (1,2),(2,1)(1,2),(2,1)(1,2),(2,1)
  • ac=3ac=3ac=3: (1,3),(3,1)(1,3),(3,1)(1,3),(3,1)

Count = 1+2+2=51+2+2=51+2+2=5.


Case 5: b=5b=5b=5

b2=25b^2=25b2=25 Need: 4ac<25⇒ac<6.254ac<25 \Rightarrow ac<6.254ac<25⇒ac<6.25 So possible integer products are: ac=1,2,3,4,5,6.ac=1,2,3,4,5,6.ac=1,2,3,4,5,6. Ordered pairs:

  • ac=1ac=1ac=1: (1,1)(1,1)(1,1)
  • ac=2ac=2ac=2: (1,2),(2,1)(1,2),(2,1)(1,2),(2,1)
  • ac=3ac=3ac=3: (1,3),(3,1)(1,3),(3,1)(1,3),(3,1)
  • ac=4ac=4ac=4: (1,4),(2,2),(4,1)(1,4),(2,2),(4,1)(1,4),(2,2),(4,1)
  • ac=5ac=5ac=5: (1,5),(5,1)(1,5),(5,1)(1,5),(5,1)
  • ac=6ac=6ac=6: (1,6),(2,3),(3,2),(6,1)(1,6),(2,3),(3,2),(6,1)(1,6),(2,3),(3,2),(6,1)

Count = 1+2+2+3+2+4=141+2+2+3+2+4=141+2+2+3+2+4=14.


Case 6: b=6b=6b=6

b2=36b^2=36b2=36 Need: 4ac<36⇒ac<94ac<36 \Rightarrow ac<94ac<36⇒ac<9 So possible integer products are: ac=1,2,3,4,5,6,8.ac=1,2,3,4,5,6,8.ac=1,2,3,4,5,6,8. (Note: ac=7ac=7ac=7 is not possible with factors in the set.)

Ordered pairs:

  • ac=1ac=1ac=1: (1,1)(1,1)(1,1)
  • ac=2ac=2ac=2: (1,2),(2,1)(1,2),(2,1)(1,2),(2,1)
  • ac=3ac=3ac=3: (1,3),(3,1)(1,3),(3,1)(1,3),(3,1)
  • ac=4ac=4ac=4: (1,4),(2,2),(4,1)(1,4),(2,2),(4,1)(1,4),(2,2),(4,1)
  • ac=5ac=5ac=5: (1,5),(5,1)(1,5),(5,1)(1,5),(5,1)
  • ac=6ac=6ac=6: (1,6),(2,3),(3,2),(6,1)(1,6),(2,3),(3,2),(6,1)(1,6),(2,3),(3,2),(6,1)
  • ac=8ac=8ac=8: (2,4),(4,2)(2,4),(4,2)(2,4),(4,2)

Count = 1+2+2+3+2+4+2=161+2+2+3+2+4+2=161+2+2+3+2+4+2=16.


  1. Total favorable triples: 0+0+3+5+14+16=38.0+0+3+5+14+16=38.0+0+3+5+14+16=38.

Thus, p=38216.p=\frac{38}{216}.p=21638​. Therefore, 216p=38.216p=38.216p=38.

  1. So the correct option is 38.\boxed{38}.38​.

This matches Option A.

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