Interpret the region
We need the area of
{ ( x , y ) : ∣ x − 5 ∣ ≤ y ≤ 4 x } . \{(x,y): |x-5| \le y \le 4\sqrt{x}\}. {( x , y ) : ∣ x − 5∣ ≤ y ≤ 4 x } .
So the region lies between the curves
y = ∣ x − 5 ∣ and y = 4 x , y=|x-5| \quad \text{and} \quad y=4\sqrt{x}, y = ∣ x − 5∣ and y = 4 x ,
with the upper curve y = 4 x y=4\sqrt{x} y = 4 x and lower curve y = ∣ x − 5 ∣ y=|x-5| y = ∣ x − 5∣ .
Hence, area exists for those x x x where
∣ x − 5 ∣ ≤ 4 x , x ≥ 0. |x-5| \le 4\sqrt{x}, \qquad x\ge 0. ∣ x − 5∣ ≤ 4 x , x ≥ 0.
Find the points of intersection
Solve
∣ x − 5 ∣ = 4 x . |x-5|=4\sqrt{x}. ∣ x − 5∣ = 4 x .
Let t = x t=\sqrt{x} t = x , so x = t 2 x=t^2 x = t 2 with t ≥ 0 t\ge 0 t ≥ 0 . Then
∣ t 2 − 5 ∣ = 4 t . |t^2-5|=4t. ∣ t 2 − 5∣ = 4 t .
We solve in two cases.
Case 1: t 2 ≥ 5 t^2\ge 5 t 2 ≥ 5
Then
t 2 − 5 = 4 t t^2-5=4t t 2 − 5 = 4 t
t 2 − 4 t − 5 = 0 t^2-4t-5=0 t 2 − 4 t − 5 = 0
( t − 5 ) ( t + 1 ) = 0. (t-5)(t+1)=0. ( t − 5 ) ( t + 1 ) = 0.
Since t ≥ 0 t\ge 0 t ≥ 0 , we get t = 5 t=5 t = 5 , hence
x = t 2 = 25. x=t^2=25. x = t 2 = 25.
Case 2: t 2 < 5 t^2<5 t 2 < 5
Then
5 − t 2 = 4 t 5-t^2=4t 5 − t 2 = 4 t
t 2 + 4 t − 5 = 0 t^2+4t-5=0 t 2 + 4 t − 5 = 0
( t − 1 ) ( t + 5 ) = 0. (t-1)(t+5)=0. ( t − 1 ) ( t + 5 ) = 0.
Since t ≥ 0 t\ge 0 t ≥ 0 , we get t = 1 t=1 t = 1 , hence
x = t 2 = 1. x=t^2=1. x = t 2 = 1.
So the curves intersect at x = 1 x=1 x = 1 and x = 25 x=25 x = 25 .
Set up the area integral
Thus,
A = ∫ 1 25 ( 4 x − ∣ x − 5 ∣ ) d x . A=\int_1^{25} \left(4\sqrt{x}-|x-5|\right)\,dx. A = ∫ 1 25 ( 4 x − ∣ x − 5∣ ) d x .
Because of the absolute value, split at x = 5 x=5 x = 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 = ∫ 1 5 ( 4 x − ( 5 − x ) ) d x + ∫ 5 25 ( 4 x − ( x − 5 ) ) d x . A=\int_1^5 \left(4\sqrt{x}-(5-x)\right)dx+\int_5^{25}\left(4\sqrt{x}-(x-5)\right)dx. A = ∫ 1 5 ( 4 x − ( 5 − x ) ) d x + ∫ 5 25 ( 4 x − ( x − 5 ) ) d x .
That is,
A = ∫ 1 5 ( 4 x + x − 5 ) d x + ∫ 5 25 ( 4 x − x + 5 ) d x . A=\int_1^5 (4\sqrt{x}+x-5)\,dx+\int_5^{25}(4\sqrt{x}-x+5)\,dx. A = ∫ 1 5 ( 4 x + x − 5 ) d x + ∫ 5 25 ( 4 x − x + 5 ) d x .
Evaluate the first integral
I 1 = ∫ 1 5 ( 4 x + x − 5 ) d x . I_1=\int_1^5 (4\sqrt{x}+x-5)\,dx. I 1 = ∫ 1 5 ( 4 x + x − 5 ) d x .
Antiderivative:
∫ 4 x d x = 4 ⋅ 2 3 x 3 / 2 = 8 3 x 3 / 2 , \int 4\sqrt{x}\,dx=4\cdot \frac{2}{3}x^{3/2}=\frac{8}{3}x^{3/2}, ∫ 4 x d x = 4 ⋅ 3 2 x 3/2 = 3 8 x 3/2 ,
∫ x d x = x 2 2 , \int x\,dx=\frac{x^2}{2}, ∫ x d x = 2 x 2 ,
∫ ( − 5 ) d x = − 5 x . \int (-5)\,dx=-5x. ∫ ( − 5 ) d x = − 5 x .
So
I 1 = [ 8 3 x 3 / 2 + x 2 2 − 5 x ] 1 5 . I_1=\left[\frac{8}{3}x^{3/2}+\frac{x^2}{2}-5x\right]_1^5. I 1 = [ 3 8 x 3/2 + 2 x 2 − 5 x ] 1 5 .
At x = 5 x=5 x = 5 :
8 3 ( 5 5 ) + 25 2 − 25 = 40 5 3 − 25 2 . \frac{8}{3}(5\sqrt{5})+\frac{25}{2}-25=\frac{40\sqrt{5}}{3}-\frac{25}{2}. 3 8 ( 5 5 ) + 2 25 − 25 = 3 40 5 − 2 25 .
At x = 1 x=1 x = 1 :
8 3 + 1 2 − 5 = 8 3 − 9 2 = − 11 6 . \frac{8}{3}+\frac{1}{2}-5=\frac{8}{3}-\frac{9}{2}=-\frac{11}{6}. 3 8 + 2 1 − 5 = 3 8 − 2 9 = − 6 11 .
Thus
I 1 = 40 5 3 − 25 2 + 11 6 = 40 5 3 − 32 3 . I_1=\frac{40\sqrt{5}}{3}-\frac{25}{2}+\frac{11}{6}
=\frac{40\sqrt{5}}{3}-\frac{32}{3}. I 1 = 3 40 5 − 2 25 + 6 11 = 3 40 5 − 3 32 .
Evaluate the second integral
I 2 = ∫ 5 25 ( 4 x − x + 5 ) d x . I_2=\int_5^{25}(4\sqrt{x}-x+5)\,dx. I 2 = ∫ 5 25 ( 4 x − x + 5 ) d x .
Antiderivative:
8 3 x 3 / 2 − x 2 2 + 5 x . \frac{8}{3}x^{3/2}-\frac{x^2}{2}+5x. 3 8 x 3/2 − 2 x 2 + 5 x .
So
I 2 = [ 8 3 x 3 / 2 − x 2 2 + 5 x ] 5 25 . I_2=\left[\frac{8}{3}x^{3/2}-\frac{x^2}{2}+5x\right]_5^{25}. I 2 = [ 3 8 x 3/2 − 2 x 2 + 5 x ] 5 25 .
At x = 25 x=25 x = 25 :
8 3 ( 125 ) − 625 2 + 125 = 1000 3 − 375 2 = 875 6 . \frac{8}{3}(125)-\frac{625}{2}+125
=\frac{1000}{3}-\frac{375}{2}
=\frac{875}{6}. 3 8 ( 125 ) − 2 625 + 125 = 3 1000 − 2 375 = 6 875 .
At x = 5 x=5 x = 5 :
40 5 3 − 25 2 + 25 = 40 5 3 + 25 2 . \frac{40\sqrt{5}}{3}-\frac{25}{2}+25
=\frac{40\sqrt{5}}{3}+\frac{25}{2}. 3 40 5 − 2 25 + 25 = 3 40 5 + 2 25 .
Thus
I 2 = 875 6 − 40 5 3 − 25 2 = 400 3 − 40 5 3 . I_2=\frac{875}{6}-\frac{40\sqrt{5}}{3}-\frac{25}{2}
=\frac{400}{3}-\frac{40\sqrt{5}}{3}. I 2 = 6 875 − 3 40 5 − 2 25 = 3 400 − 3 40 5 .
Compute total area
A = I 1 + I 2 = ( 40 5 3 − 32 3 ) + ( 400 3 − 40 5 3 ) . 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 = I 1 + I 2 = ( 3 40 5 − 3 32 ) + ( 3 400 − 3 40 5 ) .
The irrational terms cancel:
A = − 32 + 400 3 = 368 3 . A=\frac{-32+400}{3}=\frac{368}{3}. A = 3 − 32 + 400 = 3 368 .
Therefore,
3 A = 368. 3A=368. 3 A = 368.
Comparison with stored answer
Stored correct answer: 368 368 368
Derived answer: 368 368 368
So they agree.