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Area Under the Curves question

2025 · 4 Apr · Shift 1 · Q49
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  5. /2025 · 4 Apr · Shift 1 · Q49

Area Under the Curves question

2025 · 4 Apr · Shift 1 · Q49

JEE MainMathematicsArea Under the CurvesNumerical+4 / −1
If the area of the region {(x,y):∣x−5∣≤y≤4x}\{(x, y):|x-5| \leq y \leq 4 \sqrt{x}\}{(x,y):∣x−5∣≤y≤4x​} is AAA, then 3A3 A3A is equal to ‾\underline{\hspace{2cm}}​.
Numerical answer
View written solutionFree

Correct answer: 368

  1. Interpret the region

We need the area of

{(x,y):∣x−5∣≤y≤4x}.\{(x,y): |x-5| \le y \le 4\sqrt{x}\}.{(x,y):∣x−5∣≤y≤4x​}.

So the region lies between the curves

y=∣x−5∣andy=4x,y=|x-5| \quad \text{and} \quad y=4\sqrt{x},y=∣x−5∣andy=4x​,

with the upper curve y=4xy=4\sqrt{x}y=4x​ and lower curve y=∣x−5∣y=|x-5|y=∣x−5∣.

Hence, area exists for those xxx where

∣x−5∣≤4x,x≥0.|x-5| \le 4\sqrt{x}, \qquad x\ge 0.∣x−5∣≤4x​,x≥0.
  1. Find the points of intersection

Solve

∣x−5∣=4x.|x-5|=4\sqrt{x}.∣x−5∣=4x​.

Let t=xt=\sqrt{x}t=x​, so x=t2x=t^2x=t2 with t≥0t\ge 0t≥0. Then

∣t2−5∣=4t.|t^2-5|=4t.∣t2−5∣=4t.

We solve in two cases.

Case 1: t2≥5t^2\ge 5t2≥5

Then

t2−5=4tt^2-5=4tt2−5=4t t2−4t−5=0t^2-4t-5=0t2−4t−5=0 (t−5)(t+1)=0.(t-5)(t+1)=0.(t−5)(t+1)=0.

Since t≥0t\ge 0t≥0, we get t=5t=5t=5, hence

x=t2=25.x=t^2=25.x=t2=25.

Case 2: t2<5t^2<5t2<5

Then

5−t2=4t5-t^2=4t5−t2=4t t2+4t−5=0t^2+4t-5=0t2+4t−5=0 (t−1)(t+5)=0.(t-1)(t+5)=0.(t−1)(t+5)=0.

Since t≥0t\ge 0t≥0, we get t=1t=1t=1, hence

x=t2=1.x=t^2=1.x=t2=1.

So the curves intersect at x=1x=1x=1 and x=25x=25x=25.


  1. Set up the area integral

Thus,

A=∫125(4x−∣x−5∣) dx.A=\int_1^{25} \left(4\sqrt{x}-|x-5|\right)\,dx.A=∫125​(4x​−∣x−5∣)dx.

Because of the absolute value, split at x=5x=5x=5:

∣x−5∣={5−x,1≤x≤5,x−5,5≤x≤25.|x-5|= \begin{cases} 5-x, & 1\le x\le 5,\\ x-5, & 5\le x\le 25. \end{cases}∣x−5∣={5−x,x−5,​1≤x≤5,5≤x≤25.​

Hence

A=∫15(4x−(5−x))dx+∫525(4x−(x−5))dx.A=\int_1^5 \left(4\sqrt{x}-(5-x)\right)dx+\int_5^{25}\left(4\sqrt{x}-(x-5)\right)dx.A=∫15​(4x​−(5−x))dx+∫525​(4x​−(x−5))dx.

That is,

A=∫15(4x+x−5) dx+∫525(4x−x+5) dx.A=\int_1^5 (4\sqrt{x}+x-5)\,dx+\int_5^{25}(4\sqrt{x}-x+5)\,dx.A=∫15​(4x​+x−5)dx+∫525​(4x​−x+5)dx.
  1. Evaluate the first integral
I1=∫15(4x+x−5) dx.I_1=\int_1^5 (4\sqrt{x}+x-5)\,dx.I1​=∫15​(4x​+x−5)dx.

Antiderivative:

∫4x dx=4⋅23x3/2=83x3/2,\int 4\sqrt{x}\,dx=4\cdot \frac{2}{3}x^{3/2}=\frac{8}{3}x^{3/2},∫4x​dx=4⋅32​x3/2=38​x3/2, ∫x dx=x22,\int x\,dx=\frac{x^2}{2},∫xdx=2x2​, ∫(−5) dx=−5x.\int (-5)\,dx=-5x.∫(−5)dx=−5x.

So

I1=[83x3/2+x22−5x]15.I_1=\left[\frac{8}{3}x^{3/2}+\frac{x^2}{2}-5x\right]_1^5.I1​=[38​x3/2+2x2​−5x]15​.

At x=5x=5x=5:

83(55)+252−25=4053−252.\frac{8}{3}(5\sqrt{5})+\frac{25}{2}-25=\frac{40\sqrt{5}}{3}-\frac{25}{2}.38​(55​)+225​−25=3405​​−225​.

At x=1x=1x=1:

83+12−5=83−92=−116.\frac{8}{3}+\frac{1}{2}-5=\frac{8}{3}-\frac{9}{2}=-\frac{11}{6}.38​+21​−5=38​−29​=−611​.

Thus

I1=4053−252+116=4053−323.I_1=\frac{40\sqrt{5}}{3}-\frac{25}{2}+\frac{11}{6} =\frac{40\sqrt{5}}{3}-\frac{32}{3}.I1​=3405​​−225​+611​=3405​​−332​.
  1. Evaluate the second integral
I2=∫525(4x−x+5) dx.I_2=\int_5^{25}(4\sqrt{x}-x+5)\,dx.I2​=∫525​(4x​−x+5)dx.

Antiderivative:

83x3/2−x22+5x.\frac{8}{3}x^{3/2}-\frac{x^2}{2}+5x.38​x3/2−2x2​+5x.

So

I2=[83x3/2−x22+5x]525.I_2=\left[\frac{8}{3}x^{3/2}-\frac{x^2}{2}+5x\right]_5^{25}.I2​=[38​x3/2−2x2​+5x]525​.

At x=25x=25x=25:

83(125)−6252+125=10003−3752=8756.\frac{8}{3}(125)-\frac{625}{2}+125 =\frac{1000}{3}-\frac{375}{2} =\frac{875}{6}.38​(125)−2625​+125=31000​−2375​=6875​.

At x=5x=5x=5:

4053−252+25=4053+252.\frac{40\sqrt{5}}{3}-\frac{25}{2}+25 =\frac{40\sqrt{5}}{3}+\frac{25}{2}.3405​​−225​+25=3405​​+225​.

Thus

I2=8756−4053−252=4003−4053.I_2=\frac{875}{6}-\frac{40\sqrt{5}}{3}-\frac{25}{2} =\frac{400}{3}-\frac{40\sqrt{5}}{3}.I2​=6875​−3405​​−225​=3400​−3405​​.
  1. Compute total area
A=I1+I2=(4053−323)+(4003−4053).A=I_1+I_2 =\left(\frac{40\sqrt{5}}{3}-\frac{32}{3}\right)+\left(\frac{400}{3}-\frac{40\sqrt{5}}{3}\right).A=I1​+I2​=(3405​​−332​)+(3400​−3405​​).

The irrational terms cancel:

A=−32+4003=3683.A=\frac{-32+400}{3}=\frac{368}{3}.A=3−32+400​=3368​.

Therefore,

3A=368.3A=368.3A=368.
  1. Comparison with stored answer

Stored correct answer: 368368368

Derived answer: 368368368

So they agree.

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