ò01 x-x dx
By Pooya Farshim (P2572) on Saturday,
June 3, 2000 - 08:05 pm :
Prove that:
I tried the trapezium rule but got stuck in the mid-way!
By Michael Doré (P904) on Wednesday, June 7,
2000 - 02:03 pm :
How about this?
Let I(a,b)=òxa ln(x)b dx
(By the way all integrals here are with respect to x and between 0 and 1.)
Integration by parts reveals that:
I(a,b)=(xa+1 ln(x)b)/(a+1)-b/(a+1)I(a,b-1)
Provided a+1 ³ b the first term on the right hand side will be zero after
the limits are applied, so we can then say:
I(a,b)=-b/(a+1)I(a,b-1)
By successive application of this formula we can evaluate I(n,n) (for +ve
integral n) as:
I(n,n)=(-n)(-n+1)(-n+2)...(-2)(-1)/(n+1)n×I(n,0) Clearly I(n,0)=1/(n+1)
So I(n,n)=(-1)n n!/(n+1)n+1
Now back to the problem.
òx-x dx can be written as òe-x ln x dx which can now be
expanded out using the infinite series. We get
ò[1-x ln x+(x ln x)2/2!-(x ln x)3/3!+...] dx
=I(0,0)-I(1,1)+I(2,2)/2!-I(3,3)/3!+...
Now the factorials will cancel with the factorials contained within the
I(n,n)s. The (-1)n will disappear and we are left with:
1-1+2-2+3-3+...
as required.
Yours,
Michael
By Pooya Farshim (P2572) on Wednesday,
June 14, 2000 - 09:33 pm :
Thanks Michael