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
- A.both (S1) and (S2) are true
- B.both (S1) and (S2) are falsecorrect
- C.only (S2) is true
- 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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