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Let $$\Omega$$ be the sample space and $$\mathrm{A \subseteq \Omega}$$ be an event. Given below are two statements : (S1) : If P(A) = 0, then A = $$\phi$$ (S2) : If P(A) = 1, then A = $$\Omega$$ Then :

JEE · Math · previous-year question

  1. A.both (S1) and (S2) are true
  2. B.both (S1) and (S2) are falsecorrect
  3. C.only (S2) is true
  4. D.only (S1) is true

Answer

B. both (S1) and (S2) are false

Explanation

$\Omega=$ sample space $\mathrm{A}=$ be an event $ \Omega$ = A wire of length 1 which starts at point 0 and ends at point 1 on the coordinate axis = $[0,1]$ $\mathrm{A}=\left\{\frac{1}{2}\right\}$ = Selecting a point on the wire which is at $\left\{\frac{1} {2}\right\}$ or 0.5 As wire is an 1-D object so from geometrical probability $P(A)=\frac{\text { Favourable Length }}{\text { Total Length }}$ Here total length of wire = 1 unit and point has zero length so point A at $\left\{\frac{1} {2}\right\}$ or 0.5 has length = 0. $$ \therefore $$ Favorable length = 0 $$ \therefore $$ $\mathrm{P}(\mathrm{A})=0 {\text { but }} \mathrm{A} \neq \phi$ Now $\overline{\mathrm{A}}$ = $[0,1]$ - $\left\{\frac{1} {2}\right\}$ So, length of $\overline{\mathrm{A}}$ = Length of entire wire - Length of point A = 1 $$ \therefore $$ $\mathrm{P}(\overline{\mathrm{A}})=1 {\text { but }} \overline{\mathrm{A}} \neq \Omega$. Then both statements are false. Attention : According to NTA option A is correct. Which is wrong. That is proven here using geometrical probability. Note : Geometrical probability : 1. For 1-D object, $P(A)=\frac{\text { Favourable Length }}{\text { Total Length }}$ 2. For 2-D object, $P(A)=\frac{\text { Favourable Area }}{\text { Total Area }}$ 3. For 2-D object, $P(A)=\frac{\text { Favourable volume }}{\text { Total volume }}$

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