- #1
swampwiz
- 571
- 83
I am looking at the formulae for the integral of csch( x ) at Wikipedia.
http://en.wikipedia.org/wiki/List_of_integrals_of_hyperbolic_functions
and it seems that there is a mistake, specially the solution of (presume a is 1 here for simplicity)
ln | [ ( cosh( x ) - 1 ] / sinh( x ) ]
while there is also
ln | sinh( x ) / [ ( cosh( x ) + 1 ] |
the latter of which I get. The issue is that the solution to this latter one results in the former, only with terms swapped around the division operator.
Here is my derivation for them:
csch( x ) = csch2( x ) / [ csch2( x ) ]1/2
= csch2( x ) / [ coth2( x ) - 1 ]1/2
u = coth( x )
du = - csch2( x )
csch( x ) dx = - ( u2 - 1 )( - 1/2 )
∫ - ( u2 - 1 )( - 1/2 ) = - ln | u + ( u2 - 1 )1/2 | + C
= - ln | coth( x ) + [ coth2( x ) - 1 ]1/2 | + C
= - ln | coth( x ) + [ { csch2( x ) } ]1/2 | + C = - ln | coth( x ) + csch( x ) | + C
= - ln | [ cosh( x ) / sinh( x ) ] + [ 1 / sinh( x ) ] | + C
= - ln | [ cosh( x ) + 1 ] / sinh( x ) | + C
= ln | sinh( x ) / [ cosh( x ) + 1 ] | + C
So I get one the solutions, but not the other, which as I had said, seems to have the terms swapped around the division operator.
I presume that it is proper to consider the square root of something to be +.
http://en.wikipedia.org/wiki/List_of_integrals_of_hyperbolic_functions
and it seems that there is a mistake, specially the solution of (presume a is 1 here for simplicity)
ln | [ ( cosh( x ) - 1 ] / sinh( x ) ]
while there is also
ln | sinh( x ) / [ ( cosh( x ) + 1 ] |
the latter of which I get. The issue is that the solution to this latter one results in the former, only with terms swapped around the division operator.
Here is my derivation for them:
csch( x ) = csch2( x ) / [ csch2( x ) ]1/2
= csch2( x ) / [ coth2( x ) - 1 ]1/2
u = coth( x )
du = - csch2( x )
csch( x ) dx = - ( u2 - 1 )( - 1/2 )
∫ - ( u2 - 1 )( - 1/2 ) = - ln | u + ( u2 - 1 )1/2 | + C
= - ln | coth( x ) + [ coth2( x ) - 1 ]1/2 | + C
= - ln | coth( x ) + [ { csch2( x ) } ]1/2 | + C = - ln | coth( x ) + csch( x ) | + C
= - ln | [ cosh( x ) / sinh( x ) ] + [ 1 / sinh( x ) ] | + C
= - ln | [ cosh( x ) + 1 ] / sinh( x ) | + C
= ln | sinh( x ) / [ cosh( x ) + 1 ] | + C
So I get one the solutions, but not the other, which as I had said, seems to have the terms swapped around the division operator.
I presume that it is proper to consider the square root of something to be +.