ANSWER π 27 ⋅ ( 10 10 − 1 ) ≅ 3.5631 \frac{\pi}{27}\cdot (10\sqrt{10}-1)\cong3.5631 27 π ⋅ ( 10 10 − 1 ) ≅ 3.5631
EXPLANTION
If f ′ ( x ) f'(x) f ′ ( x ) is continuous on [ a , b ] [a,b] [ a , b ] and the curve y = f ( x ) y=f(x) y = f ( x ) is rotated about x-axis , the area of the surface of revolution so generated is
S = 2 π ∫ a b ∣ f ( x ) ∣ 1 + ( f ′ ( x ) ) 2 d x S =2\pi \int_{a}^{b}\left | f(x) \right |\sqrt{1+\left ( f'(x) \right )^{2}}dx S = 2 π ∫ a b ∣ f ( x ) ∣ 1 + ( f ′ ( x ) ) 2 d x
Let f ( x ) = x 3 f(x)=x^3 f ( x ) = x 3 , a = 0 , b = 1 a=0, b=1 a = 0 , b = 1 , then f ′ ( x ) = 3 x 2 f'(x)=3x^2 f ′ ( x ) = 3 x 2 and
S = 2 π ∫ 0 1 x 3 1 + 9 x 4 d x S =2\pi \int_{0}^{1}x^3\sqrt{1+9x^4}dx S = 2 π ∫ 0 1 x 3 1 + 9 x 4 d x
Calculate the integral using the substitution 1 + 9 x 4 = t 1+9x^4=t 1 + 9 x 4 = t . So, since 36 x 3 d x = d t 36x^3dx=dt 36 x 3 d x = d t , we have
S = 2 π 36 ∫ 0 1 36 x 3 1 + 9 x 4 d x = = π 18 ∫ 1 10 t d t = 2 π 18 ⋅ 3 ⋅ [ t 3 2 ] 1 10 = 2 π 18 ⋅ 3 ⋅ 10 10 − 2 π 18 ⋅ 3 = π 27 ⋅ ( 10 10 − 1 ) ≅ 3.5631 S = \frac{2\pi}{36}\int_{0}^{1}36x^3\sqrt{1+9x^4}dx=\\= \frac{\pi}{18} \int_{1}^{10}\sqrt{t}dt=\frac{2\pi}{18\cdot 3}\cdot[t^{\frac{3}{2}}]_{1} ^{10} =\frac{2\pi}{18\cdot 3}\cdot10\sqrt{10} -\frac{2\pi}{18\cdot 3} = \frac{\pi}{27}\cdot (10\sqrt{10}-1)\cong3.5631 S = 36 2 π ∫ 0 1 36 x 3 1 + 9 x 4 d x = = 18 π ∫ 1 10 t d t = 18 ⋅ 3 2 π ⋅ [ t 2 3 ] 1 10 = 18 ⋅ 3 2 π ⋅ 10 10 − 18 ⋅ 3 2 π = 27 π ⋅ ( 10 10 − 1 ) ≅ 3.5631
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