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Area Under the Curves question

2023 · 31 Jan · Shift 2 · Q38
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  5. /2023 · 31 Jan · Shift 2 · Q38

Area Under the Curves question

2023 · 31 Jan · Shift 2 · Q38

JEE MainMathematicsArea Under the CurvesNumerical+4 / −1
Let the area of the region {(x,y):∣2x−1∣≤y≤∣x2−x∣,0≤x≤1}\left\{(x, y):|2 x-1| \leq y \leq\left|x^{2}-x\right|, 0 \leq x \leq 1\right\}{(x,y):∣2x−1∣≤y≤​x2−x​,0≤x≤1} be A\mathrm{A}A. Then (6 A+11)2(6 \mathrm{~A}+11)^{2}(6 A+11)2 is equal to
Numerical answer
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Correct answer: 125

  1. We need the area of the region
{(x,y):∣2x−1∣≤y≤∣x2−x∣, 0≤x≤1}.\{(x,y): |2x-1| \le y \le |x^2-x|,\, 0\le x\le 1\}.{(x,y):∣2x−1∣≤y≤∣x2−x∣,0≤x≤1}.

So for each x∈[0,1]x\in[0,1]x∈[0,1], the vertical thickness is

∣x2−x∣−∣2x−1∣,|x^2-x|-|2x-1|,∣x2−x∣−∣2x−1∣,

but only where the upper curve is above the lower curve.

  1. Simplify the expressions on [0,1][0,1][0,1]:
  • Since x∈[0,1]x\in[0,1]x∈[0,1], we have
x2−x=x(x−1)≤0x^2-x=x(x-1)\le 0x2−x=x(x−1)≤0

so

∣x2−x∣=−(x2−x)=x−x2=x(1−x).|x^2-x|=-(x^2-x)=x-x^2=x(1-x).∣x2−x∣=−(x2−x)=x−x2=x(1−x).
  • Also,
∣2x−1∣={1−2x,0≤x≤12,2x−1,12≤x≤1.|2x-1|= \begin{cases} 1-2x, & 0\le x\le \tfrac12,\\ 2x-1, & \tfrac12\le x\le 1. \end{cases}∣2x−1∣={1−2x,2x−1,​0≤x≤21​,21​≤x≤1.​

Thus we need to solve

∣2x−1∣≤x−x2.|2x-1| \le x-x^2.∣2x−1∣≤x−x2.
  1. Case 1: 0≤x≤120\le x\le \tfrac120≤x≤21​

Then ∣2x−1∣=1−2x|2x-1|=1-2x∣2x−1∣=1−2x. Inequality becomes

1−2x≤x−x2.1-2x \le x-x^2.1−2x≤x−x2.

Rearrange:

0≤3x−x2−10\le 3x-x^2-10≤3x−x2−1 x2−3x+1≤0.x^2-3x+1\le 0.x2−3x+1≤0.

Its roots are

3±52.\frac{3\pm\sqrt{5}}{2}.23±5​​.

Hence in [0,12][0,\tfrac12][0,21​], the valid part is

x∈[3−52,12].x\in\left[\frac{3-\sqrt5}{2},\frac12\right].x∈[23−5​​,21​].
  1. Case 2: 12≤x≤1\tfrac12\le x\le 121​≤x≤1

Then ∣2x−1∣=2x−1|2x-1|=2x-1∣2x−1∣=2x−1. Inequality becomes

2x−1≤x−x2.2x-1 \le x-x^2.2x−1≤x−x2.

Rearrange:

0≤1−x−x20\le 1-x-x^20≤1−x−x2 x2+x−1≤0.x^2+x-1\le 0.x2+x−1≤0.

Its roots are

−1±52.\frac{-1\pm\sqrt5}{2}.2−1±5​​.

Hence in [12,1][\tfrac12,1][21​,1], the valid part is

x∈[12,5−12].x\in\left[\frac12,\frac{\sqrt5-1}{2}\right].x∈[21​,25​−1​].

So overall the region exists for

x∈[3−52,5−12].x\in\left[\frac{3-\sqrt5}{2},\frac{\sqrt5-1}{2}\right].x∈[23−5​​,25​−1​].
  1. Compute the area by splitting at x=12x=\tfrac12x=21​:
A=∫(3−5)/21/2[(x−x2)−(1−2x)]dx+∫1/2(5−1)/2[(x−x2)−(2x−1)]dx.A=\int_{(3-\sqrt5)/2}^{1/2}\Big[(x-x^2)-(1-2x)\Big]dx +\int_{1/2}^{(\sqrt5-1)/2}\Big[(x-x^2)-(2x-1)\Big]dx.A=∫(3−5​)/21/2​[(x−x2)−(1−2x)]dx+∫1/2(5​−1)/2​[(x−x2)−(2x−1)]dx.

Simplify integrands:

(x−x2)−(1−2x)=3x−x2−1,(x-x^2)-(1-2x)=3x-x^2-1,(x−x2)−(1−2x)=3x−x2−1, (x−x2)−(2x−1)=1−x−x2.(x-x^2)-(2x-1)=1-x-x^2.(x−x2)−(2x−1)=1−x−x2.

Thus

A=∫(3−5)/21/2(3x−x2−1) dx+∫1/2(5−1)/2(1−x−x2) dx.A=\int_{(3-\sqrt5)/2}^{1/2}(3x-x^2-1)\,dx +\int_{1/2}^{(\sqrt5-1)/2}(1-x-x^2)\,dx.A=∫(3−5​)/21/2​(3x−x2−1)dx+∫1/2(5​−1)/2​(1−x−x2)dx.
  1. A symmetry substitution makes this easier. Let
u=x−12.u=x-\frac12.u=x−21​.

Then

∣2x−1∣=2∣u∣,|2x-1|=2|u|,∣2x−1∣=2∣u∣,

and

x−x2=14−u2.x-x^2=\frac14-u^2.x−x2=41​−u2.

So the condition becomes

2∣u∣≤14−u2.2|u|\le \frac14-u^2.2∣u∣≤41​−u2.

For u≥0u\ge 0u≥0:

2u≤14−u2  ⟺  u2+2u−14≤0.2u\le \frac14-u^2 \iff u^2+2u-\frac14\le 0.2u≤41​−u2⟺u2+2u−41​≤0.

The positive endpoint is

a=5−22.a=\frac{\sqrt5-2}{2}.a=25​−2​.

Hence

A=2∫0a(14−u2−2u)du.A=2\int_0^a \left(\frac14-u^2-2u\right)du.A=2∫0a​(41​−u2−2u)du.

Now,

∫(14−u2−2u)du=u4−u33−u2.\int \left(\frac14-u^2-2u\right)du=\frac{u}{4}-\frac{u^3}{3}-u^2.∫(41​−u2−2u)du=4u​−3u3​−u2.

So

A=2[u4−u33−u2]0a.A=2\left[\frac{u}{4}-\frac{u^3}{3}-u^2\right]_0^a.A=2[4u​−3u3​−u2]0a​.

With

a=5−22,a=\frac{\sqrt5-2}{2},a=25​−2​,

this simplifies to

A=55−116.A=\frac{5\sqrt5-11}{6}.A=655​−11​.
  1. Now compute:
6A+11=6⋅55−116+11=55.6A+11 = 6\cdot \frac{5\sqrt5-11}{6}+11 = 5\sqrt5.6A+11=6⋅655​−11​+11=55​.

Therefore

(6A+11)2=(55)2=125.(6A+11)^2=(5\sqrt5)^2=125.(6A+11)2=(55​)2=125.

So the required integer is

125.\boxed{125}.125​.
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