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Area Under the Curves question

2025 · 2 Apr · Shift 1 · Q48
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Area Under the Curves question

2025 · 2 Apr · Shift 1 · Q48

JEE MainMathematicsArea Under the CurvesNumerical+4 / −1
If the area of the region {(x,y):∣4−x2∣≤y≤x2,y≤4,x≥0}\left\{(x, y):\left|4-x^2\right| \leq y \leq x^2, y \leq 4, x \geq 0\right\}{(x,y):​4−x2​≤y≤x2,y≤4,x≥0} is (802α−β),α,β∈N\left(\frac{80 \sqrt{2}}{\alpha}-\beta\right), \alpha, \beta \in \mathbf{N}(α802​​−β),α,β∈N, then α+β\alpha+\betaα+β is equal to ‾\underline{\hspace{2cm}}​.
Numerical answer
View written solutionFree

Correct answer: 22

  1. Given region

We need the area of

{(x,y):∣4−x2∣≤y≤x2, y≤4, x≥0}.\{(x,y): |4-x^2|\le y\le x^2,\ y\le 4,\ x\ge 0\}.{(x,y):∣4−x2∣≤y≤x2, y≤4, x≥0}.

So for each x≥0x\ge 0x≥0, the allowed yyy must satisfy:

  • lower bound: y≥∣4−x2∣y\ge |4-x^2|y≥∣4−x2∣
  • upper bounds: y≤x2y\le x^2y≤x2 and y≤4y\le 4y≤4

Hence the actual upper bound is

y≤min⁡(x2,4).y\le \min(x^2,4).y≤min(x2,4).

So the vertical strip exists when

∣4−x2∣≤min⁡(x2,4).|4-x^2|\le \min(x^2,4).∣4−x2∣≤min(x2,4).
  1. Split according to xxx

Because of x2x^2x2 and 444, the natural breakpoints are around x=2x=2x=2.

Case 1: 0≤x≤20\le x\le 20≤x≤2

Then x2≤4x^2\le 4x2≤4, so

min⁡(x2,4)=x2.\min(x^2,4)=x^2.min(x2,4)=x2.

Also,

∣4−x2∣=4−x2|4-x^2|=4-x^2∣4−x2∣=4−x2

since x2≤4x^2\le 4x2≤4.

Thus the strip is

4−x2≤y≤x2.4-x^2\le y\le x^2.4−x2≤y≤x2.

This is possible only if

4−x2≤x2  ⟹  4≤2x2  ⟹  x2≥2  ⟹  x≥2.4-x^2\le x^2 \implies 4\le 2x^2 \implies x^2\ge 2 \implies x\ge \sqrt{2}.4−x2≤x2⟹4≤2x2⟹x2≥2⟹x≥2​.

So in this range, valid xxx are

2≤x≤2.\sqrt{2}\le x\le 2.2​≤x≤2.

The height of the strip is

x2−(4−x2)=2x2−4.x^2-(4-x^2)=2x^2-4.x2−(4−x2)=2x2−4.

Case 2: x≥2x\ge 2x≥2

Then x2≥4x^2\ge 4x2≥4, so

min⁡(x2,4)=4.\min(x^2,4)=4.min(x2,4)=4.

Also,

∣4−x2∣=x2−4.|4-x^2|=x^2-4.∣4−x2∣=x2−4.

Thus the strip is

x2−4≤y≤4.x^2-4\le y\le 4.x2−4≤y≤4.

This is possible only if

x2−4≤4  ⟹  x2≤8  ⟹  x≤22.x^2-4\le 4 \implies x^2\le 8 \implies x\le 2\sqrt{2}.x2−4≤4⟹x2≤8⟹x≤22​.

So in this range, valid xxx are

2≤x≤22.2\le x\le 2\sqrt{2}.2≤x≤22​.

The height of the strip is

4−(x2−4)=8−x2.4-(x^2-4)=8-x^2.4−(x2−4)=8−x2.
  1. Set up the area integral

Therefore,

A=∫22(2x2−4) dx+∫222(8−x2) dx.A=\int_{\sqrt{2}}^{2}(2x^2-4)\,dx+\int_{2}^{2\sqrt{2}}(8-x^2)\,dx.A=∫2​2​(2x2−4)dx+∫222​​(8−x2)dx.
  1. Evaluate the first integral
∫(2x2−4)dx=2x33−4x.\int (2x^2-4)dx=\frac{2x^3}{3}-4x.∫(2x2−4)dx=32x3​−4x.

So

∫22(2x2−4)dx=[2x33−4x]22.\int_{\sqrt{2}}^{2}(2x^2-4)dx =\left[\frac{2x^3}{3}-4x\right]_{\sqrt{2}}^{2}.∫2​2​(2x2−4)dx=[32x3​−4x]2​2​.

At x=2x=2x=2:

2(8)3−8=163−8=−83.\frac{2(8)}{3}-8=\frac{16}{3}-8=-\frac{8}{3}.32(8)​−8=316​−8=−38​.

At x=2x=\sqrt{2}x=2​:

2(2)33−42=2(22)3−42=423−42=−823.\frac{2(\sqrt{2})^3}{3}-4\sqrt{2} =\frac{2(2\sqrt{2})}{3}-4\sqrt{2} =\frac{4\sqrt{2}}{3}-4\sqrt{2} =-\frac{8\sqrt{2}}{3}.32(2​)3​−42​=32(22​)​−42​=342​​−42​=−382​​.

Therefore,

I1=−83−(−823)=82−83.I_1=-\frac{8}{3}-\left(-\frac{8\sqrt{2}}{3}\right) =\frac{8\sqrt{2}-8}{3}.I1​=−38​−(−382​​)=382​−8​.
  1. Evaluate the second integral
∫(8−x2)dx=8x−x33.\int (8-x^2)dx=8x-\frac{x^3}{3}.∫(8−x2)dx=8x−3x3​.

So

∫222(8−x2)dx=[8x−x33]222.\int_{2}^{2\sqrt{2}}(8-x^2)dx =\left[8x-\frac{x^3}{3}\right]_{2}^{2\sqrt{2}}.∫222​​(8−x2)dx=[8x−3x3​]222​​.

At x=22x=2\sqrt{2}x=22​:

8(22)−(22)33=162−1623=3223.8(2\sqrt{2})-\frac{(2\sqrt{2})^3}{3} =16\sqrt{2}-\frac{16\sqrt{2}}{3} =\frac{32\sqrt{2}}{3}.8(22​)−3(22​)3​=162​−3162​​=3322​​.

At x=2x=2x=2:

16−83=403.16-\frac{8}{3}=\frac{40}{3}.16−38​=340​.

Therefore,

I2=3223−403.I_2=\frac{32\sqrt{2}}{3}-\frac{40}{3}.I2​=3322​​−340​.
  1. Total area
A=I1+I2=82−83+322−403=402−483.A=I_1+I_2 =\frac{8\sqrt{2}-8}{3}+\frac{32\sqrt{2}-40}{3} =\frac{40\sqrt{2}-48}{3}.A=I1​+I2​=382​−8​+3322​−40​=3402​−48​.

So

A=4023−16.A=\frac{40\sqrt{2}}{3}-16.A=3402​​−16.

This matches the form

802α−β.\frac{80\sqrt{2}}{\alpha}-\beta.α802​​−β.

Thus,

802α=4023  ⟹  α=6,\frac{80\sqrt{2}}{\alpha}=\frac{40\sqrt{2}}{3} \implies \alpha=6,α802​​=3402​​⟹α=6,

and

β=16.\beta=16.β=16.

Hence,

α+β=6+16=22.\alpha+\beta=6+16=22.α+β=6+16=22.
  1. Comparison with stored answer

Derived answer = 222222. Stored correct answer = 222222.

They agree.

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