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Functions question

2022 · 24 Jun · Shift 1 · Q35
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Functions question

2022 · 24 Jun · Shift 1 · Q35

JEE MainMathematicsFunctionsNumerical+4 / −1
The number of one-one functions f : {a, b, c, d} →\to→{0, 1, 2, ......, 10} such that 2f(a) −-− f(b) + 3f(c) + f(d) = 0 is ‾\underline{\hspace{2cm}}​.
Numerical answer
View written solutionFree

Correct answer: 31

  1. Let x=f(a),y=f(b),z=f(c),w=f(d).x=f(a),\quad y=f(b),\quad z=f(c),\quad w=f(d).x=f(a),y=f(b),z=f(c),w=f(d). Then x,y,z,w∈{0,1,2,…,10}x,y,z,w\in\{0,1,2,\dots,10\}x,y,z,w∈{0,1,2,…,10} and since fff is one-one, x,y,z,wx,y,z,wx,y,z,w are all distinct.

  2. The given condition is 2f(a)−f(b)+3f(c)+f(d)=0,2f(a)-f(b)+3f(c)+f(d)=0,2f(a)−f(b)+3f(c)+f(d)=0, i.e. 2x−y+3z+w=0.2x-y+3z+w=0.2x−y+3z+w=0. Rearranging, y=2x+3z+w.y=2x+3z+w.y=2x+3z+w.

  3. Since x,z,w≥0x,z,w\ge 0x,z,w≥0, we have y=2x+3z+w≥0.y=2x+3z+w\ge 0.y=2x+3z+w≥0. Also y≤10y\le 10y≤10 because codomain is {0,1,…,10}\{0,1,\dots,10\}{0,1,…,10}.

    So we must count ordered triples (x,z,w)(x,z,w)(x,z,w) of distinct elements from {0,1,…,10}\{0,1,\dots,10\}{0,1,…,10} such that 2x+3z+w≤10,2x+3z+w\le 10,2x+3z+w≤10, and then set y=2x+3z+wy=2x+3z+wy=2x+3z+w with the additional requirement that yyy is distinct from x,z,wx,z,wx,z,w.

  4. Because of the coefficient 3z3z3z, we must have small values of zzz.


Case 1: z=0z=0z=0

Then y=2x+wy=2x+wy=2x+w with distinctness requiring x,w,0x,w,0x,w,0 distinct, so x,w≠0x,w\ne 0x,w=0 and x≠wx\ne wx=w. Also y≤10y\le 10y≤10.

Now count ordered pairs (x,w)(x,w)(x,w) with x,w∈{1,2,…,10}x,w\in\{1,2,\dots,10\}x,w∈{1,2,…,10}, x≠wx\ne wx=w, and 2x+w≤10.2x+w\le 10.2x+w≤10.

For each xxx:

  • x=1x=1x=1: w≤8w\le 8w≤8, with w∈{1,2,…,8}w\in\{1,2,\dots,8\}w∈{1,2,…,8} and w≠1w\ne 1w=1 gives 777 values.
  • x=2x=2x=2: w≤6w\le 6w≤6, from {1,2,…,6}\{1,2,\dots,6\}{1,2,…,6} excluding 222 gives 555 values.
  • x=3x=3x=3: w≤4w\le 4w≤4, from {1,2,3,4}\{1,2,3,4\}{1,2,3,4} excluding 333 gives 333 values.
  • x=4x=4x=4: w≤2w\le 2w≤2, from {1,2}\{1,2\}{1,2} gives 222 values.
  • x≥5x\ge 5x≥5: no solution.

Total so far: 7+5+3+2=17.7+5+3+2=17.7+5+3+2=17.

But we must also ensure y≠x,w,0y\ne x,w,0y=x,w,0.

  • y=0y=0y=0 impossible since 2x+w>02x+w>02x+w>0.
  • y=xy=xy=x would mean 2x+w=x⇒x+w=02x+w=x\Rightarrow x+w=02x+w=x⇒x+w=0, impossible.
  • y=wy=wy=w would mean 2x+w=w⇒x=02x+w=w\Rightarrow x=02x+w=w⇒x=0, impossible.

So all 171717 are valid.


Case 2: z=1z=1z=1

Then y=2x+w+3.y=2x+w+3.y=2x+w+3. Distinctness requires x,w≠1x,w\ne 1x,w=1, x≠wx\ne wx=w. Also 2x+w+3≤10⇒2x+w≤7.2x+w+3\le 10\Rightarrow 2x+w\le 7.2x+w+3≤10⇒2x+w≤7.

Now xxx cannot be 000? It can be, since only distinct from z=1z=1z=1 and www. So count ordered pairs (x,w)(x,w)(x,w) with x,w∈{0,1,…,10}x,w\in\{0,1,\dots,10\}x,w∈{0,1,…,10}, x,w≠1x,w\ne 1x,w=1, x≠wx\ne wx=w, and 2x+w≤72x+w\le 72x+w≤7.

For each xxx:

  • x=0x=0x=0: w≤7w\le 7w≤7, w∈{0,2,3,4,5,6,7}w\in\{0,2,3,4,5,6,7\}w∈{0,2,3,4,5,6,7} excluding w=0w=0w=0 gives 666 values.
  • x=2x=2x=2: w≤3w\le 3w≤3, w∈{0,2,3}w\in\{0,2,3\}w∈{0,2,3} excluding 222 gives 222 values.
  • x=3x=3x=3: w≤1w\le 1w≤1, possible w=0w=0w=0 only, so 111 value.
  • x≥4x\ge 4x≥4: no solution.

So tentative total: 6+2+1=9.6+2+1=9.6+2+1=9.

Now check y≠x,w,1y\ne x,w,1y=x,w,1.

  • y=1y=1y=1 impossible since 2x+w+3≥32x+w+3\ge 32x+w+3≥3.
  • y=xy=xy=x gives 2x+w+3=x⇒x+w+3=02x+w+3=x\Rightarrow x+w+3=02x+w+3=x⇒x+w+3=0, impossible.
  • y=wy=wy=w gives 2x+w+3=w⇒2x+3=02x+w+3=w\Rightarrow 2x+3=02x+w+3=w⇒2x+3=0, impossible.

Hence all 999 are valid.


Case 3: z=2z=2z=2

Then y=2x+w+6,y=2x+w+6,y=2x+w+6, so 2x+w≤4.2x+w\le 4.2x+w≤4. Also x,w≠2x,w\ne 2x,w=2, x≠wx\ne wx=w.

Count ordered pairs:

  • x=0x=0x=0: w≤4w\le 4w≤4, from {0,1,3,4}\{0,1,3,4\}{0,1,3,4} excluding 000 gives 333 values.
  • x=1x=1x=1: w≤2w\le 2w≤2, from {0,1}\{0,1\}{0,1} excluding 111 gives 111 value.
  • x≥2x\ge 2x≥2: no solution.

Tentative total: 3+1=4.3+1=4.3+1=4.

Check y≠x,w,2y\ne x,w,2y=x,w,2.

  • y=2y=2y=2 impossible.
  • y=xy=xy=x impossible.
  • y=wy=wy=w would imply 2x+6=02x+6=02x+6=0, impossible.

So all 444 are valid.


Case 4: z≥3z\ge 3z≥3

Then 3z≥9.3z\ge 9.3z≥9. Since x,w≥0x,w\ge 0x,w≥0, y=2x+3z+w≥9.y=2x+3z+w\ge 9.y=2x+3z+w≥9. But if z=3z=3z=3, then 2x+w≤12x+w\le 12x+w≤1. Since x,wx,wx,w must be distinct and neither can equal 333, only possibility is (x,w)=(0,1)(x,w)=(0,1)(x,w)=(0,1) or (1,0)(1,0)(1,0). Let us check:

  • (x,w)=(0,1)(x,w)=(0,1)(x,w)=(0,1) gives y=10y=10y=10, valid and distinct.
  • (x,w)=(1,0)(x,w)=(1,0)(x,w)=(1,0) gives y=11y=11y=11, not allowed.

So for z=3z=3z=3, there is exactly 111 solution.

If z≥4z\ge 4z≥4, then y≥12>10,y\ge 12>10,y≥12>10, so no solution.

Thus Case 4 contributes 111.


  1. Total number of one-one functions is 17+9+4+1=31.17+9+4+1=31.17+9+4+1=31.

Therefore, the required number is 31.\boxed{31}.31​.

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