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Circle question

2022 · 29 Jul · Shift 2 · Q43
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Circle question

2022 · 29 Jul · Shift 2 · Q43

JEE MainMathematicsCircleNumerical+4 / −1
 Let S={(x,y)∈N×N:9(x−3)2+16(y−4)2≤144}\text { Let } S=\left\{(x, y) \in \mathbb{N} \times \mathbb{N}: 9(x-3)^{2}+16(y-4)^{2} \leq 144\right\} Let S={(x,y)∈N×N:9(x−3)2+16(y−4)2≤144} and T={(x,y)∈R×R:(x−7)2+(y−4)2≤36}T=\left\{(x, y) \in \mathbb{R} \times \mathbb{R}:(x-7)^{2}+(y-4)^{2} \leq 36\right\}T={(x,y)∈R×R:(x−7)2+(y−4)2≤36}. Then n(S∩T)n(S \cap T)n(S∩T) is equal to ‾\underline{\hspace{2cm}}​.
Numerical answer
View written solutionFree

Correct answer: 27

  1. Interpret the two regions

We are given S={(x,y)∈N×N:9(x−3)2+16(y−4)2≤144}S=\{(x,y)\in \mathbb N\times \mathbb N: 9(x-3)^2+16(y-4)^2\le 144\}S={(x,y)∈N×N:9(x−3)2+16(y−4)2≤144} and T={(x,y)∈R×R:(x−7)2+(y−4)2≤36}.T=\{(x,y)\in \mathbb R\times \mathbb R:(x-7)^2+(y-4)^2\le 36\}.T={(x,y)∈R×R:(x−7)2+(y−4)2≤36}.

We need the number of points in S∩TS\cap TS∩T.

Since S⊂N×NS\subset \mathbb N\times \mathbb NS⊂N×N, the intersection S∩TS\cap TS∩T consists of natural-number lattice points satisfying both inequalities.


  1. Rewrite the inequalities in standard form

For SSS: 9(x−3)2+16(y−4)2≤1449(x-3)^2+16(y-4)^2\le 1449(x−3)2+16(y−4)2≤144 Divide by 144144144: (x−3)216+(y−4)29≤1.\frac{(x-3)^2}{16}+\frac{(y-4)^2}{9}\le 1.16(x−3)2​+9(y−4)2​≤1.

So SSS is the set of natural-number points inside/on the ellipse centered at (3,4)(3,4)(3,4) with semi-axes 444 and 333.

For TTT: (x−7)2+(y−4)2≤36,(x-7)^2+(y-4)^2\le 36,(x−7)2+(y−4)2≤36, which is the disk centered at (7,4)(7,4)(7,4) with radius 666.


  1. Find possible natural values of xxx from the ellipse

From (x−3)216+(y−4)29≤1,\frac{(x-3)^2}{16}+\frac{(y-4)^2}{9}\le 1,16(x−3)2​+9(y−4)2​≤1, we must have (x−3)216≤1  ⟹  (x−3)2≤16.\frac{(x-3)^2}{16}\le 1 \implies (x-3)^2\le 16.16(x−3)2​≤1⟹(x−3)2≤16. Thus −4≤x−3≤4  ⟹  −1≤x≤7.-4\le x-3\le 4 \implies -1\le x\le 7.−4≤x−3≤4⟹−1≤x≤7. Since x∈Nx\in \mathbb Nx∈N, possible values are x=1,2,3,4,5,6,7.x=1,2,3,4,5,6,7.x=1,2,3,4,5,6,7.

Now also impose the circle condition. Since the circle is centered at x=7x=7x=7 with radius 666, (x−7)2≤36  ⟹  1≤x≤13,(x-7)^2\le 36 \implies 1\le x\le 13,(x−7)2≤36⟹1≤x≤13, so all x=1x=1x=1 to 777 are allowed in principle.

Hence we check each integer x=1,2,3,4,5,6,7x=1,2,3,4,5,6,7x=1,2,3,4,5,6,7.


  1. For each xxx, find allowed integer yyy values

We need both 9(x−3)2+16(y−4)2≤144and(x−7)2+(y−4)2≤36.9(x-3)^2+16(y-4)^2\le 144 \quad \text{and} \quad (x-7)^2+(y-4)^2\le 36.9(x−3)2+16(y−4)2≤144and(x−7)2+(y−4)2≤36.

Because y∈Ny\in \mathbb Ny∈N, we count natural-number yyy satisfying both.

Let k=(y−4)2k=(y-4)^2k=(y−4)2.


Case 1: x=1x=1x=1

Ellipse: 9(1−3)2+16(y−4)2≤1449(1-3)^2+16(y-4)^2\le 1449(1−3)2+16(y−4)2≤144 9⋅4+16(y−4)2≤1449\cdot 4+16(y-4)^2\le 1449⋅4+16(y−4)2≤144 36+16(y−4)2≤14436+16(y-4)^2\le 14436+16(y−4)2≤144 16(y−4)2≤10816(y-4)^2\le 10816(y−4)2≤108 (y−4)2≤274=6.75.(y-4)^2\le \frac{27}{4}=6.75.(y−4)2≤427​=6.75.

So ∣y−4∣≤2|y-4|\le 2∣y−4∣≤2, hence y=2,3,4,5,6.y=2,3,4,5,6.y=2,3,4,5,6.

Circle: (1−7)2+(y−4)2≤36(1-7)^2+(y-4)^2\le 36(1−7)2+(y−4)2≤36 36+(y−4)2≤3636+(y-4)^2\le 3636+(y−4)2≤36 (y−4)2≤0,(y-4)^2\le 0,(y−4)2≤0, so y=4.y=4.y=4.

Thus for x=1x=1x=1, count =1=1=1.


Case 2: x=2x=2x=2

Ellipse: 9(2−3)2+16(y−4)2≤1449(2-3)^2+16(y-4)^2\le 1449(2−3)2+16(y−4)2≤144 9+16(y−4)2≤1449+16(y-4)^2\le 1449+16(y−4)2≤144 16(y−4)2≤13516(y-4)^2\le 13516(y−4)2≤135 (y−4)2≤13516=8.4375.(y-4)^2\le \frac{135}{16}=8.4375.(y−4)2≤16135​=8.4375. So ∣y−4∣≤2|y-4|\le 2∣y−4∣≤2, hence y=2,3,4,5,6.y=2,3,4,5,6.y=2,3,4,5,6.

Circle: (2−7)2+(y−4)2≤36(2-7)^2+(y-4)^2\le 36(2−7)2+(y−4)2≤36 25+(y−4)2≤3625+(y-4)^2\le 3625+(y−4)2≤36 (y−4)2≤11.(y-4)^2\le 11.(y−4)2≤11. So ∣y−4∣≤3|y-4|\le 3∣y−4∣≤3, giving y=1,2,3,4,5,6,7.y=1,2,3,4,5,6,7.y=1,2,3,4,5,6,7.

Common values: y=2,3,4,5,6.y=2,3,4,5,6.y=2,3,4,5,6. Count =5=5=5.


Case 3: x=3x=3x=3

Ellipse: 16(y−4)2≤144  ⟹  (y−4)2≤9,16(y-4)^2\le 144 \implies (y-4)^2\le 9,16(y−4)2≤144⟹(y−4)2≤9, so y=1,2,3,4,5,6,7.y=1,2,3,4,5,6,7.y=1,2,3,4,5,6,7.

Circle: (3−7)2+(y−4)2≤36(3-7)^2+(y-4)^2\le 36(3−7)2+(y−4)2≤36 16+(y−4)2≤3616+(y-4)^2\le 3616+(y−4)2≤36 (y−4)2≤20.(y-4)^2\le 20.(y−4)2≤20. So ∣y−4∣≤4|y-4|\le 4∣y−4∣≤4, giving y=1,2,3,4,5,6,7,8.y=1,2,3,4,5,6,7,8.y=1,2,3,4,5,6,7,8.

Common values: y=1,2,3,4,5,6,7.y=1,2,3,4,5,6,7.y=1,2,3,4,5,6,7. Count =7=7=7.


Case 4: x=4x=4x=4

Ellipse: 9(4−3)2+16(y−4)2≤1449(4-3)^2+16(y-4)^2\le 1449(4−3)2+16(y−4)2≤144 9+16(y−4)2≤1449+16(y-4)^2\le 1449+16(y−4)2≤144 (y−4)2≤13516,(y-4)^2\le \frac{135}{16},(y−4)2≤16135​, so y=2,3,4,5,6.y=2,3,4,5,6.y=2,3,4,5,6.

Circle: (4−7)2+(y−4)2≤36(4-7)^2+(y-4)^2\le 36(4−7)2+(y−4)2≤36 9+(y−4)2≤369+(y-4)^2\le 369+(y−4)2≤36 (y−4)2≤27,(y-4)^2\le 27,(y−4)2≤27, so ∣y−4∣≤5|y-4|\le 5∣y−4∣≤5, hence y=1,2,3,4,5,6,7,8,9.y=1,2,3,4,5,6,7,8,9.y=1,2,3,4,5,6,7,8,9.

Common values: y=2,3,4,5,6.y=2,3,4,5,6.y=2,3,4,5,6. Count =5=5=5.


Case 5: x=5x=5x=5

Ellipse: 9(5−3)2+16(y−4)2≤1449(5-3)^2+16(y-4)^2\le 1449(5−3)2+16(y−4)2≤144 36+16(y−4)2≤14436+16(y-4)^2\le 14436+16(y−4)2≤144 16(y−4)2≤10816(y-4)^2\le 10816(y−4)2≤108 (y−4)2≤6.75,(y-4)^2\le 6.75,(y−4)2≤6.75, so y=2,3,4,5,6.y=2,3,4,5,6.y=2,3,4,5,6.

Circle: (5−7)2+(y−4)2≤36(5-7)^2+(y-4)^2\le 36(5−7)2+(y−4)2≤36 4+(y−4)2≤364+(y-4)^2\le 364+(y−4)2≤36 (y−4)2≤32,(y-4)^2\le 32,(y−4)2≤32, so ∣y−4∣≤5|y-4|\le 5∣y−4∣≤5, hence y=1,2,3,4,5,6,7,8,9.y=1,2,3,4,5,6,7,8,9.y=1,2,3,4,5,6,7,8,9.

Common values: y=2,3,4,5,6.y=2,3,4,5,6.y=2,3,4,5,6. Count =5=5=5.


Case 6: x=6x=6x=6

Ellipse: 9(6−3)2+16(y−4)2≤1449(6-3)^2+16(y-4)^2\le 1449(6−3)2+16(y−4)2≤144 81+16(y−4)2≤14481+16(y-4)^2\le 14481+16(y−4)2≤144 16(y−4)2≤6316(y-4)^2\le 6316(y−4)2≤63 (y−4)2≤6316=3.9375.(y-4)^2\le \frac{63}{16}=3.9375.(y−4)2≤1663​=3.9375. So ∣y−4∣≤1|y-4|\le 1∣y−4∣≤1, hence y=3,4,5.y=3,4,5.y=3,4,5.

Circle: (6−7)2+(y−4)2≤36(6-7)^2+(y-4)^2\le 36(6−7)2+(y−4)2≤36 1+(y−4)2≤361+(y-4)^2\le 361+(y−4)2≤36 (y−4)2≤35,(y-4)^2\le 35,(y−4)2≤35, so ∣y−4∣≤5|y-4|\le 5∣y−4∣≤5, hence y=1,2,3,4,5,6,7,8,9.y=1,2,3,4,5,6,7,8,9.y=1,2,3,4,5,6,7,8,9.

Common values: y=3,4,5.y=3,4,5.y=3,4,5. Count =3=3=3.


Case 7: x=7x=7x=7

Ellipse: 9(7−3)2+16(y−4)2≤1449(7-3)^2+16(y-4)^2\le 1449(7−3)2+16(y−4)2≤144 144+16(y−4)2≤144144+16(y-4)^2\le 144144+16(y−4)2≤144 (y−4)2≤0,(y-4)^2\le 0,(y−4)2≤0, so y=4.y=4.y=4.

Circle: (7−7)2+(y−4)2≤36(7-7)^2+(y-4)^2\le 36(7−7)2+(y−4)2≤36 (y−4)2≤36,(y-4)^2\le 36,(y−4)2≤36, so yyy can be many values, but intersection gives only y=4.y=4.y=4.

Count =1=1=1.


  1. Add all counts

Total number of points is 1+5+7+5+5+3+1=27.1+5+7+5+5+3+1=27.1+5+7+5+5+3+1=27.

Therefore, n(S∩T)=27.n(S\cap T)=27.n(S∩T)=27.


  1. Comparison with stored answer

Stored correct answer: 272727

Our derived answer is also 272727, so it agrees.

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