Evaluate the following:
∫ 0 ∞ x 5 x 3 − sin 3 x d x
If the result can be expressed as c a π b where a and c are coprime. Find a + b + c .
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Pff..i did what you said in the hint..that was long indeed..
can you please give the solution by your second method
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@Aman Kumar I've added the second solution.
I don't get this part: "Note that f(0)=f'(0)=... then: f(a)=..." Then what's 39pi/4?
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f ( 4 ) ( 0 ) isn't needed, we have f ( 3 ) ( a ) = 3 2 1 3 π a + C , set a = 0 to get C = 0 and repeat this process.
Did using the first method.Nice solution!
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For a ≥ 0 define : f ( a ) = ∫ 0 ∞ x 5 ( a x ) 3 − sin 3 ( a x ) d x Differentiate it 4 times (note that the third derivative is absolutely convergent, then we can differentiate) to get : f ( 4 ) ( a ) = ∫ 0 ∞ 4 x 8 1 sin ( 3 a x ) − 4 x 3 sin ( a x ) d x = 2 π ( 4 8 1 − 4 3 ) = 4 3 9 π . Note that
f ( 0 ) = f ′ ( 0 ) = f ( 2 ) ( 0 ) = f ( 3 ) ( 0 ) = 0 then : f ( a ) = 3 2 1 3 a 4 π
Easily we get f ( 1 ) = 3 2 1 3 π , then answer is 1 3 + 1 + 3 2 = 4 6 .
Another method : we have I = 4 1 ∫ 0 ∞ x 5 4 x 3 + sin ( 3 x ) − 3 sin x d x Integrate by parts to get : 4 I = ∫ 0 ∞ x 5 4 x 3 + sin ( 3 x ) − 3 sin x d x = 4 − 1 = 0 [ x 4 4 x 3 + sin ( 3 x ) − 3 sin x ] 0 ∞ + 4 1 ∫ 0 ∞ x 4 1 2 x 2 − 3 cos x + 3 cos ( 3 x ) d x The brackets gives zero since near zero we have 4 x 3 + sin ( 3 x ) − 3 sin x = 2 x 5 + o ( x 5 ) by Taylor series, and the limit at ∞ is obviously 0 , we repeat the same thing 1 6 I = ∫ 0 ∞ x 4 1 2 x 2 − 3 cos x + 3 cos ( 3 x ) d x = 3 1 ∫ 0 ∞ x 3 2 4 x + 3 sin x − 9 sin 3 x d x And again two times : 4 8 I = 2 1 ∫ 0 ∞ x 2 2 4 + 3 cos x − 2 7 cos ( 3 x ) d x = 2 1 ∫ 0 ∞ x 8 1 sin ( 3 x ) − x 3 sin x d x We know that for any a = 0 ∫ 0 ∞ x sin a x d x = 2 π (For a proof Google for Dirichlet integral), then : 9 6 I = ( 8 1 − 3 ) 2 π ⟹ I = 3 2 1 3 π .