Porisma

Worked solution

Compare two slice directions and translate an axis of rotation

Designer A programs a machine to cut a thin material into the region enclosed by y=2x−3y=\sqrt{2x-3}, 2y=x−42y=x-4 and the x-axis.

(i) Sketch suitable graphs and find its exact surface area. marks

(ii) Rotate that region through four right angles about the x-axis to form a closed object. Find its volume. marks

(iii) Designer B rotates the region bounded by C:y=x2+3C:y=x^2+3, x=4x=4 and both axes through four right angles about x=4x=4. Write the equation of CC translated four units in the negative x-direction, and determine the resulting volume. marks

[2018/MI/Promo/PU2/I/11]

CJC · Review problems · Question 16

Answer

Area 50/350/3; (ii) corrected 875π/12≈229875\pi/12\approx229 cubic units; (iii) translated curve y=(x+4)2+3y=(x+4)^2+3, volume 272π/3≈285272\pi/3\approx285.

(i) Locate the meeting point and use horizontal strips

Write the boundaries as x=(y2+3)/2x=(y^2+3)/2 and x=2y+4x=2y+4. Equality gives y2−4y−5=0y^2-4y-5=0; the square-root graph requires y≥0y\ge0, so y=5y=5 and x=14x=14. The x-intercepts are 3/23/2 and 44. Area =∫05[(2y+4)−(y2+3)/2]dy=[y2−y3/6+5y/2]05=50/3=\int_0^5[(2y+4)-(y^2+3)/2]dy=[y^2-y^3/6+5y/2]_0^5=50/3 square units.

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(i) Alternative: subtract two vertical areas

The same area is ∫3/2142x−3 dx−∫414(x/2−2)dx=[(2x−3)3/2/3]3/214−[x2/4−2x]414=50/3\int_{3/2}^{14}\sqrt{2x-3}\,dx-\int_4^{14}(x/2-2)dx=[(2x-3)^{3/2}/3]_{3/2}^{14}-[x^2/4-2x]_4^{14}=50/3.

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(ii) Method 1: subtract the inner washers

V=π∫3/214(2x−3)dx−π∫414(x/2−2)2dx=π(625/4−250/3)=875π/12≈229.07446V=\pi\int_{3/2}^{14}(2x-3)dx-\pi\int_4^{14}(x/2-2)^2dx=\pi(625/4-250/3)=875\pi/12\approx229.07446, or 229229 cubic units (3 s.f.). The source prints the same integral setup but reports 222222; that numerical evaluation is incorrect.

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(ii) Method 2: subtract a cone

The full outer solid has volume 625π/4625\pi/4. The removed cone has radius 55 and height 14−4=1014-4=10, so its volume is 13π(52)(10)=250π/3\tfrac13\pi(5^2)(10)=250\pi/3. Their difference is again 875π/12875\pi/12, confirming the corrected result independently of numerical quadrature.

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(iii) Translate the curve and preserve the relevant branch

After shifting the original x-coordinate left by 44, the curve is y=(x+4)2+3y=(x+4)^2+3, with the region now on −4≤x≤0-4\le x\le0 and rotation axis x=0x=0. Its right-side inverse branch is x=y−3−4x=\sqrt{y-3}-4 for 3≤y≤193\le y\le19.

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(iii) Add the base cylinder and variable-radius disks

The source setup is V=π(42)(3)+π∫319(y−3−4)2dyV=\pi(4^2)(3)+\pi\int_3^{19}(\sqrt{y-3}-4)^2dy. Evaluation gives 272π/3≈284.83773=285272\pi/3\approx284.83773=285 cubic units (3 s.f.). Shells in the original coordinates, 2π∫04(4−x)(x2+3)dx2\pi\int_0^4(4-x)(x^2+3)dx, verify the same result. The radius above y=3y=3 is 4−y−34-\sqrt{y-3}; squaring the negative translated coordinate gives its square.

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Compare your sketch

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Two panels: Designer A’s region lies above the x-axis from x=3/2 to 4, then between sqrt(2x−3) and x/2−2 up to (14,5). Designer B’s translated region lies under y=(x+4)²+3 on −4≤x≤0, with the y-axis as its rotation axis.
Two panels: Designer A’s region lies above the x-axis from x=3/2 to 4, then between sqrt(2x−3) and x/2−2 up to (14,5). Designer B’s translated region lies under y=(x+4)²+3 on −4≤x≤0, with the y-axis as its rotation axis.
Two panels: Designer A’s region lies above the x-axis from x=3/2 to 4, then between sqrt(2x−3) and x/2−2 up to (14,5). Designer B’s translated region lies under y=(x+4)²+3 on −4≤x≤0, with the y-axis as its rotation axis.

Common mistakes

The printed 222 contradicts both source volume methods; their actual value is 875 pi / 12, about 229.

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