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 tanx↦x:
f(t)=∫∞0sintx1+x2dx,
then, taking the Laplace transform:
L{f}(s)=∫∞011+x2∫∞0e−stsin(xt)dtdx=∫∞011+x2Im[∫∞0e−(s−ix)tdt]dx=∫∞011+x2xs2+x2dx=12∫∞0dx(1+x)(s2+x)=12s2−2∫∞0[11+x−1s2+x]dx=lnss2−1.
Now, I see that f(t)=L−1{lnss2−1}(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)(x−i)dx
f(t)=i2(∫∞0sin(tx)x+idx−∫∞0sin(tx)x−idx)
Consider now
I(a)=∫sin(tx)x+adx=∫sin(t(y−a))ydy
I(a)=cos(at)∫sin(ty)ydy−sin(at)∫cos(ty)ydy Now, make z=ty
I(a)=cos(at)∫sin(z)zdz−sin(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(π−2Si(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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