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Functions question

2025 · 22 Jan · Shift 2 · Q41
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Functions question

2025 · 22 Jan · Shift 2 · Q41

JEE MainMathematicsFunctionsMCQ+4 / −1
Let A={1,2,3,4}\mathrm{A}=\{1,2,3,4\}A={1,2,3,4} and B={1,4,9,16}\mathrm{B}=\{1,4,9,16\}B={1,4,9,16}. Then the number of many-one functions f:A→Bf: \mathrm{A} \rightarrow \mathrm{B}f:A→B such that 1∈f( A)1 \in f(\mathrm{~A})1∈f( A) is equal to :
  1. A
    151
  2. B
    139
  3. C
    163
  4. D
    127
View written solutionFree

Correct answer: A

  1. We need the number of many-one functions f:A→Bf:A\to Bf:A→B such that 1∈f(A)1\in f(A)1∈f(A), where A={1,2,3,4},B={1,4,9,16}.A=\{1,2,3,4\},\qquad B=\{1,4,9,16\}.A={1,2,3,4},B={1,4,9,16}.

Here, many-one means the function is not one-one.

  1. First count all functions from AAA to BBB such that 1∈f(A)1\in f(A)1∈f(A).

Since ∣A∣=4|A|=4∣A∣=4 and ∣B∣=4|B|=4∣B∣=4, total number of functions from AAA to BBB is 44=256.4^4=256.44=256.

Now exclude those functions whose image does not contain 111. Then each element of AAA can map only to {4,9,16}\{4,9,16\}{4,9,16}, so number of such functions is 34=81.3^4=81.34=81.

Hence, number of functions with 1∈f(A)1\in f(A)1∈f(A) is 256−81=175.256-81=175.256−81=175.

  1. Among these, subtract the one-one functions, because we want many-one functions.

A one-one function from a 4-element set to a 4-element set is actually a bijection. For a bijection to satisfy 1∈f(A)1\in f(A)1∈f(A) is automatic, since every element of BBB appears exactly once.

Number of one-one functions from AAA to BBB is 4!=24.4!=24.4!=24.

  1. Therefore, required number of many-one functions is 175−24=151.175-24=151.175−24=151.

  2. Option check:

  • A: 151151151 ✅
  • B: 139139139
  • C: 163163163
  • D: 127127127

So the correct option is A.

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