∫ − ∞ ∞ x 2 + 4 x + 5 sin x d x
If the value of the above integral is equal to − π A e B sin C where A , B and C are integers, find A × B × C .
This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try
refreshing the page, (b) enabling javascript if it is disabled on your browser and,
finally, (c)
loading the
non-javascript version of this page
. We're sorry about the hassle.
∫ − ∞ ∞ x 2 + 4 x + 5 sin x d x = = = = ∫ − ∞ ∞ ( x + 2 ) 2 + 1 sin x d x = y = x + 2 ∫ − ∞ ∞ y 2 + 1 sin ( y − 2 ) d y ∫ − ∞ ∞ y 2 + 1 sin y cos 2 − cos y sin 2 d y cos 2 = 0 ( ∵ odd function ) ∫ − ∞ ∞ y 2 + 2 sin y d y − sin 2 ∫ − ∞ ∞ y 2 + 1 cos y d y − sin 2 ∫ − ∞ ∞ y 2 + 1 cos y d y
Claim : ∫ − ∞ ∞ y 2 + 1 cos y d y = e π .
Proof : Consider the Laplace transform of the function g ( a ) = ∫ 0 ∞ 1 + x 2 cos ( a x ) d x ,
L ( g ( a ) ) = ∫ 0 ∞ ∫ 0 ∞ 1 + x 2 cos ( a x ) e − a s d a d x = ∫ 0 ∞ ( 1 + x 2 ) ( s 2 + x 2 ) s d x = 2 ( s + 1 ) π .
Then g ( a ) = L − 1 ( 2 ( s + 1 ) π ) = 2 π e − ∣ a ∣ ⇒ ∫ − ∞ ∞ 1 + x 2 cos ( x ) d x = 2 g ( 1 ) = e π .
Hence, ∫ − ∞ ∞ x 2 + 4 x + 5 sin x d x = − sin 2 ⋅ e π .
Pretty cool!!
Log in to reply
Thanks. I was actually planning on doing the standard "differentiate through the integral" approach, but I thought I can spice things up!
Problem Loading...
Note Loading...
Set Loading...
It depends on what you call elementary. Here is a solution without contour integration.
The function f ( x ) = e − ∣ x ∣ has Fourier transform ( F f ) ( t ) = 2 π 1 ∫ R f ( x ) e − i x t d x = π 2 1 + t 2 1 and so, since the integrand is even, ∫ R x 2 + 1 cos x d x = 2 π ∫ R ( F f ) ( x ) e i x d x = π f ( 1 ) = e π Thus the integral we want is ∫ R x 2 + 4 x + 5 sin x d x = ∫ R x 2 + 1 sin ( x − 2 ) d x = − sin 2 ∫ R x 2 + 1 cos x d x = − π e − 1 sin 2 making the answer 1 × − 1 × 2 = − 2 .