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JEE Main 20203 SeptemberMorning Shift
Exam
JEE Main
Year
2020
Date
3 September
Shift
Morning Shift
Subject
Mathematics
Chapter
Quadratic Equation

JEE Main 2020 · 3 September · Morning Shift · Mathematics · Quadratic Equation

Consider the two sets: A = m ∈ R: both the roots of x ^ 2 - ( m + 1...

Consider the two sets:
A = { m \in R: both the roots of x2−(m+1)x+m+4=0{x}^{2} - ( m + 1 ) x + m + 4 = 0 are real } and B=[−3,5)B = [ - 3, 5 )
Which of the following is not true?

Options

  1. AA−B=(−∞,−3)∪(5,∞)A - B = ( - \infty , - 3 ) \cup ( 5, \infty )
  2. BA∩B=−3A \cap B = { - 3 }
  3. CB−A=(−3,5)B - A = ( - 3, 5 )
  4. DA∪B=RA \cup B = R

Correct answer

A. A−B=(−∞,−3)∪(5,∞)A - B = ( - \infty , - 3 ) \cup ( 5, \infty )

Step-by-step solution

Since both roots are real (m+1)2−4(m+4)≥0{\left(m + 1\right)}^{2} - 4\left(m + 4\right) \ge 0

⇒m2−2m−15≥0⇒(m−5)(m+3)≥0⇒ {m}^{2} - 2m - 15 \ge 0 ⇒ \left(m - 5\right)\left(m + 3\right) \ge 0

⇒m∈(−∞,−3]∪[5,∞)⇒ m \in ( - \infty , - 3 ] \cup [ 5, \infty )

⇒A=(−∞,−3]∪[5,∞)⇒ A = ( - \infty , - 3 ] \cup [ 5, \infty )

A−B=(−∞,−3)∪[5,∞)A - B = \left(- \infty , - 3\right) \cup [ 5, \infty )

A∩B={−3}A \cap B = \left\{- 3\right\}

B−A=(−3,5)B - A = \left(- 3, 5\right)

A∪B=RA \cup B = R

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