0 ∫ π 5 − 2 cos x x 2 d x
If the solution of the above integral can be depicted as:
b π a + 2 π ln c ( f d + e )
where a , b , c , d , e , f are integers, what is the value of a + b + c − d + e − f ?
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.
Obviously this is not an original question.(Wish i had that kind of brains)
Credit goes to china(tian27546).
Problem Loading...
Note Loading...
Set Loading...
My solution is similar, but not identical.
We have ∫ 0 1 x + a ln x d x = − 6 1 π 2 − 2 1 ln 2 a + 2 1 ln 2 ( 1 + a ) + L i 2 ( 1 + a a ) a > 0 and hence, with ϕ = 2 1 ( 5 + 1 ) being the Golden Section, ∫ 0 1 x + ϕ ln x d x ∫ 0 1 x + ϕ − 1 ln x d x = − 6 1 π 2 + 2 3 ln 2 ϕ + L i 2 ( ϕ − 1 ) = − 6 1 π 2 + L i 2 ( ϕ − 2 ) and hence ∫ 0 1 x 2 + 5 x + 1 ln x d x = ∫ 0 1 ( x + ϕ ) ( x + ϕ − 1 ) ln x d x = L i 2 ( ϕ − 2 ) − L i 2 ( ϕ − 1 ) − 2 3 ln 2 ϕ = ( 1 5 1 π 2 − 4 1 ln 2 ( ϕ − 2 ) ) − ( 1 0 1 π 2 − ln 2 ( ϕ − 1 ) ) − 2 3 ln 2 ϕ = − 3 0 1 π 2 − 2 3 ln 2 ϕ using standard properties of the dilogarithm function L i 2 .
Now consider the cut plane with the negative real axis removed, and the key-hole contour Γ = γ 1 − γ 2 − γ 3 + γ 4 , where
The function f ( z ) = z 2 − 5 z + 1 ( lo g z ) 2 is analytic inside and on the curve Γ , except for a simple pole at z = ϕ − 1 . Now ∫ γ 1 f ( z ) d z ( ∫ γ 2 − ∫ γ 4 ) f ( z ) d z ∫ γ 3 f ( z ) d z = ∫ − π π e i x ( 2 cos x − 5 ) ( i x ) 2 i e i x d x = 2 i ∫ 0 1 5 − 2 cos x x 2 d x = − ∫ 0 1 x 2 + 5 x + 1 ( ln x + π i ) 2 − ( ln x − π i ) 2 d x = − 4 π i ∫ 0 1 x 2 + 5 x + 1 ln x d x = O ( ϵ ln ϵ ) ϵ → 0 Thus, since ( ∫ γ 1 − ∫ γ 2 − ∫ γ 3 + ∫ γ 4 ) f ( z ) d z = 2 π i R e s z = ϕ 1 f ( z ) = − 2 π i ln 2 ( ϕ − 1 ) = − 2 π i ln 2 ϕ we deduce, letting ϵ → 0 , that ∫ 0 π 5 − 2 cos x x 2 d x + 2 π ∫ 0 1 x 2 + 5 x + 1 ln x d x = − π ln 2 ( ϕ − 1 ) = − π ln 2 ϕ and hence ∫ 0 π 5 − 2 cos x x 2 d x = 1 5 1 π 3 + 2 π ln 2 ϕ which makes the answer 3 + 1 5 + 2 − 1 + 5 − 2 = 2 2 .