Monday, December 10, 2018

calculus - Integral f(t)=intpi/20sin(ttanx)dx




Evaluate the integral
f(t)=π/20sin(ttanx)dx.




I came across this integral after having shown that its counterpart can be evaluated exactly:
π/20cos(ttanx)dt=π2e|t|.

I figured that f would be as easy but it is actually presenting me with some difficulties. I start off by using the substitution tanxx:
f(t)=0sintx1+x2dx,
then, taking the Laplace transform:
L{f}(s)=011+x20estsin(xt)dtdx=011+x2Im[0e(six)tdt]dx=011+x2xs2+x2dx=120dx(1+x)(s2+x)=12s220[11+x1s2+x]dx=lnss21.
Now, I see that f(t)=L1{lnss21}(t), but I do not know how to compute that inverse Laplace transform. Could I have some help?


Answer



After Kemono Chen's comment.



Starting with
f(t)=0sin(tx)1+x2dx=0sin(tx)(x+i)(xi)dx
f(t)=i2(0sin(tx)x+idx0sin(tx)xidx)
Consider now
I(a)=sin(tx)x+adx=sin(t(ya))ydy

I(a)=cos(at)sin(ty)ydysin(at)cos(ty)ydy Now, make z=ty
I(a)=cos(at)sin(z)zdzsin(at)cos(z)zdz=cos(at)Si(z)sin(at)Ci(z)
Back to x
I(a)=cos(at)Si(t(x+a))sin(at)Ci(t(x+a)) Assuming t>0
J(a)=0sin(tx)x+adx=Ci(at)sin(at)+12(π2Si(at))cos(at)
J(a)J(a)=(Ci(at)+Ci(at))sin(at)2Si(at)cos(at)
f(t)=i2(J(i)J(i))=Shi(t)cosh(t)Chi(t)sinh(t) which is Kemono Chen's result.


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